Five men and a woman suspected of terrorism offences have been arrested in London as part of an intelligence-led investigation involving MI5.
One of the six, who are aged between 18 and 30, was Tasered by police. Eight homes in west, east and north London and one business are being searched.
It is understood the arrests relate to a possible plot involving Islamist extremists, with potential UK targets.
One of those arrested in Stratford is an ex-police community support officer.
He served with the Metropolitan Police for more than two years before resigning in September 2009.
The force said he had not been "deployed in any specialist or sensitive roles."
It is understood that another of those arrested is Jahangir Alom. He was detained in Abbey Road, in Stratford, east London.
The Metropolitan Police said the arrests were not linked to the Olympics or Paralympic Games.
Security sources are describing the arrests as "significant", but there is no suggestion at the moment of any imminent attack, BBC home affairs correspondent Danny Shaw says.
It is understood that police involved in the long-running operation decided to act on Thursday on the grounds of public safety.
'Black-clad figures'
The UK terrorism threat level is unchanged from "substantial", which means a terrorist attack is a "strong possibility". There are two higher levels - "severe", meaning an attack is "highly likely", and "critical", meaning an attack is "expected imminently".
Some - if not all - of those arrested on suspicion of the commission, preparation or instigation of acts of terrorism are British nationals, our correspondent adds.
They include a 21-year-old man and a 30-year-old woman, who were detained at separate addresses in Ealing, west London, while a 29-year-old man was arrested in an Ealing street. The woman is married to one of the men.
Three brothers, aged 18, 24 and 26, were detained in Abbey Road, Stratford, east London, during the operation involving armed officers.
The 24-year-old man who was Tasered during his arrest did not need hospital treatment, police said.
All six have been taken to a police station in south-east London.
An eyewitness to the Stratford raid, Mark Window, told the BBC he had been woken up by an explosion at 04:15 BST.
"I looked out my window, saw loads of mini-explosions going off still, loads of black-clad figures milling around," he said.
"One was halfway up a ladder, through a window, and you realise it's some sort of police operation going on instead of kids messing about with fireworks which is what you think when you first wake up."
Trainee taxi driver Stephen Maguire, 23, said he heard the police from his bedroom in Eastbourne Road, which overlooks the front of the house.
"I heard the biggest bang ever and I saw a massive cloud of smoke and torches going up at the windows," he said. "It sounded like they were gunshots but they weren't."
BBC News - Six arrested in London anti-terror operation
Bus scare, terror arrests ramp up UK security concerns ahead of Olympics
London (CNN) -- Britain's police swung into action Thursday as two separate incidents, one involving a security scare on a bus and the other six terror arrests, pointed to the heightened state of alert ahead of the London Olympic Games.
Security is a central concern for UK authorities and the organizers of the Olympics, which start in three weeks.
After launching a major response to the security alert on the bus, which led armed officers to close a major motorway, police in Staffordshire concluded the incident was not terror-related.
But its officers responded "swiftly and proportionately" to credible information from a concerned member of the public, a Staffordshire police statement said.
"We can now confirm that, whilst this was a genuine security alert, the significant concerns reported to us were unfounded," the statement said. "The information received concerned a report of vapor escaping from a bag which on investigation turned out to be a health improvement aid for smokers."
Bus stopped on British motorway
Hours earlier, police in London arrested five men and a woman on terror charges, although they said the operation was not related to the Olympics.
The six, aged from 18 to 30, were all arrested on suspicion of the commission, preparation or instigation of acts of terrorism and are being questioned, London's Metropolitan Police said. Officers are searching homes in east, west and north London as well as business premises in east London.
The operation in London is not linked to the Olympic and Paralympic Games, the police said.
British security officials on alert for Olympic terror threats
Additional security measures will be in place in London and elsewhere during the Olympics.
They include the siting of missile defense systems around the capital and fighter jets on standby at air bases close to London. Police and intelligence agencies have also stepped up monitoring efforts.
Thursday's dramatic closure of the busy M6 Toll motorway, near Birmingham, England, had many hallmarks of a response to a potential terror threat and could be seen as a sign that police are taking no chances.
Armed police, firefighters and military bomb disposal units raced to the scene as passengers were evacuated from the bus, which had pulled over near the town of Lichfield. Police in Britain are not usually armed when they respond to incidents.
Officials insist no specific terror threats tied to Olympics
Passengers were lined up on the tarmac for questioning, and their luggage and the vehicle searched, before police confirmed that there was no terror threat and that no one was considered a suspect.
"Our utmost priority was the safety and security of those people on the coach and those traveling on the motorway," the force said.
The 48 passengers who were on the bus from the northwestern city of Preston to London are now being taken on to their planned destination, a spokeswoman for Megabus said.
"We would like to thank our customers for their patience and understanding during today's police operation, which followed concerns raised by a passenger during the journey," a statement from the bus operator said.
"The safety and welfare of our passengers and our staff is our absolute priority."
The bus has now been removed and the highway has reopened, police said.
Government officials from Washington to London insist there are no known specific or credible terror threats tied to the Olympic Games in London. Nonetheless, authorities on both sides of the Atlantic are urging vigilance.
The Metropolitan Police is leading the "largest peacetime operation" seen in the United Kingdom as it seeks to keep the public safe, it said, with up to 9,500 police officers from London and elsewhere to be deployed on the busiest days of the Olympics.
As a result, London's residents and its visitors can expect to see a large police presence on the streets and airport-style security at Olympic venues.
Bus scare, terror arrests ramp up UK security concerns ahead of Olympics - CNN.com
One of the six, who are aged between 18 and 30, was Tasered by police. Eight homes in west, east and north London and one business are being searched.
It is understood the arrests relate to a possible plot involving Islamist extremists, with potential UK targets.
One of those arrested in Stratford is an ex-police community support officer.
He served with the Metropolitan Police for more than two years before resigning in September 2009.
The force said he had not been "deployed in any specialist or sensitive roles."
It is understood that another of those arrested is Jahangir Alom. He was detained in Abbey Road, in Stratford, east London.
The Metropolitan Police said the arrests were not linked to the Olympics or Paralympic Games.
Security sources are describing the arrests as "significant", but there is no suggestion at the moment of any imminent attack, BBC home affairs correspondent Danny Shaw says.
It is understood that police involved in the long-running operation decided to act on Thursday on the grounds of public safety.
'Black-clad figures'
The UK terrorism threat level is unchanged from "substantial", which means a terrorist attack is a "strong possibility". There are two higher levels - "severe", meaning an attack is "highly likely", and "critical", meaning an attack is "expected imminently".
Some - if not all - of those arrested on suspicion of the commission, preparation or instigation of acts of terrorism are British nationals, our correspondent adds.
They include a 21-year-old man and a 30-year-old woman, who were detained at separate addresses in Ealing, west London, while a 29-year-old man was arrested in an Ealing street. The woman is married to one of the men.
Three brothers, aged 18, 24 and 26, were detained in Abbey Road, Stratford, east London, during the operation involving armed officers.
The 24-year-old man who was Tasered during his arrest did not need hospital treatment, police said.
All six have been taken to a police station in south-east London.
An eyewitness to the Stratford raid, Mark Window, told the BBC he had been woken up by an explosion at 04:15 BST.
"I looked out my window, saw loads of mini-explosions going off still, loads of black-clad figures milling around," he said.
"One was halfway up a ladder, through a window, and you realise it's some sort of police operation going on instead of kids messing about with fireworks which is what you think when you first wake up."
Trainee taxi driver Stephen Maguire, 23, said he heard the police from his bedroom in Eastbourne Road, which overlooks the front of the house.
"I heard the biggest bang ever and I saw a massive cloud of smoke and torches going up at the windows," he said. "It sounded like they were gunshots but they weren't."
BBC News - Six arrested in London anti-terror operation
Bus scare, terror arrests ramp up UK security concerns ahead of Olympics
London (CNN) -- Britain's police swung into action Thursday as two separate incidents, one involving a security scare on a bus and the other six terror arrests, pointed to the heightened state of alert ahead of the London Olympic Games.
Security is a central concern for UK authorities and the organizers of the Olympics, which start in three weeks.
After launching a major response to the security alert on the bus, which led armed officers to close a major motorway, police in Staffordshire concluded the incident was not terror-related.
But its officers responded "swiftly and proportionately" to credible information from a concerned member of the public, a Staffordshire police statement said.
"We can now confirm that, whilst this was a genuine security alert, the significant concerns reported to us were unfounded," the statement said. "The information received concerned a report of vapor escaping from a bag which on investigation turned out to be a health improvement aid for smokers."
Bus stopped on British motorway
Hours earlier, police in London arrested five men and a woman on terror charges, although they said the operation was not related to the Olympics.
The six, aged from 18 to 30, were all arrested on suspicion of the commission, preparation or instigation of acts of terrorism and are being questioned, London's Metropolitan Police said. Officers are searching homes in east, west and north London as well as business premises in east London.
The operation in London is not linked to the Olympic and Paralympic Games, the police said.
British security officials on alert for Olympic terror threats
Additional security measures will be in place in London and elsewhere during the Olympics.
They include the siting of missile defense systems around the capital and fighter jets on standby at air bases close to London. Police and intelligence agencies have also stepped up monitoring efforts.
Thursday's dramatic closure of the busy M6 Toll motorway, near Birmingham, England, had many hallmarks of a response to a potential terror threat and could be seen as a sign that police are taking no chances.
Armed police, firefighters and military bomb disposal units raced to the scene as passengers were evacuated from the bus, which had pulled over near the town of Lichfield. Police in Britain are not usually armed when they respond to incidents.
Officials insist no specific terror threats tied to Olympics
Passengers were lined up on the tarmac for questioning, and their luggage and the vehicle searched, before police confirmed that there was no terror threat and that no one was considered a suspect.
"Our utmost priority was the safety and security of those people on the coach and those traveling on the motorway," the force said.
The 48 passengers who were on the bus from the northwestern city of Preston to London are now being taken on to their planned destination, a spokeswoman for Megabus said.
"We would like to thank our customers for their patience and understanding during today's police operation, which followed concerns raised by a passenger during the journey," a statement from the bus operator said.
"The safety and welfare of our passengers and our staff is our absolute priority."
The bus has now been removed and the highway has reopened, police said.
Government officials from Washington to London insist there are no known specific or credible terror threats tied to the Olympic Games in London. Nonetheless, authorities on both sides of the Atlantic are urging vigilance.
The Metropolitan Police is leading the "largest peacetime operation" seen in the United Kingdom as it seeks to keep the public safe, it said, with up to 9,500 police officers from London and elsewhere to be deployed on the busiest days of the Olympics.
As a result, London's residents and its visitors can expect to see a large police presence on the streets and airport-style security at Olympic venues.
Bus scare, terror arrests ramp up UK security concerns ahead of Olympics - CNN.com
: It's a mere speculation! One should wait for the official news and then Jump on to the conclusion!
. As of now thank god that identities are not revealed
