At least 80 people have died, including children, after a man drove a truck carrying grenades and guns up to two kilometres through a crowd celebrating Bastille Day in the southern French city of Nice.
Police believe the driver, who was shot dead by police, was a 31-year-old French-Tunisian citizen.
Australia's Foreign Minister Julie Bishop has confirmed three Australians have been injured.
She said they suffered minor injuries will attempting to flee the scene.
"The Australian Government condemns the Bastille Day attack in Nice, which has left many people dead and injured," she said in an earlier statement.
"Once again we stand resolutely with the people of France. Our thoughts and sympathies are with the victims and their families."
The vision shows a white truck driving fast through the crowd as people close by could only watch in disbelief.
DFAT has provided contact numbers for anyone concerned about loved ones who may be in Nice.
"If you have any concerns for the welfare of family and friends in the region you should attempt to contact them directly," a statement said.
"If you are unable to contact them and still hold concerns for their welfare you should call the Department of Foreign Affairs and Trade 24 hour Consular Emergency Centre on
1300 555 135 or
+61 2 6261 3305 if calling from overseas."
The truck reportedly slammed into groups of revellers at speeds up to 100km/h before zig-zagging down the famous Promenade des Anglais, as panicked revellers fled in terror.
Crowds gathered in the holiday area had been watching fireworks displays.
Officials are still investigating whether the driver acted alone.
In a national address, French president Francois Hollande said 20 people had been critically injured in addition to the dead, and it could not be denied it was a terrorist attack.
If confirmed as an act of terror, the incident will be the third major attack on French soil in 18 months -- with several smaller-scale jihadist killings also having taken place.
While no group has claimed responsibility for the attack in the resort city, Hollande vowed to strengthen his country's role in the fight against the Islamic State group in Iraq and Syria.
"Nothing will make us yield in our will to fight terrorism. We will further strengthen our actions in Iraq and in Syria. We will continue striking those who attack us on our own soil," he said, in reference to the ISIL group.
Hollande said several children were among the dead after the attack, which he said was of an "undeniable terrorist nature".
He vowed ever stronger security measures -- calling up reservists and extending a state of emergency -- as he reached for familiar, and new words to boost the morale of a battered nation.
Police examine the truck. (Facebook: Sasha Goldsmith)
The truck travelled for up to two kilometres before coming to a stop. Police examine the truck. (Facebook: Sasha Goldsmith)
At least 80 people are believed dead. The truck travelled for up to two kilometres before coming to a stop. Police examine the truck. (Facebook: Sasha Goldsmith)
A spokesperson from the Topdeck tour company said one Australian from their tour group had suffered minor injuries.
"As of 2.30 local time all of our customers are safe and accounted for. Unfortunately one of our customers has reported some minor injuries," the company said in a statement.
"They are with Topdeck staff and are currently being tended to in hospital. Our family liaison team has been in direct contact with the injured customer’s family and we are ensuring the best care possible."
The bloodshed came on Bastille Day, a celebration of everything France holds dear, its secular republic and the values of "Liberte, Egalite, Fraternite" (Freedom, Equality, Fraternity).
"France was struck on its national day ... the symbol of freedom," said Hollande.
Videos and photos posted on social media show dozens of bodies strewn on the ground and scenes of mass panic as people fled the area.
A photograph showed the front of the truck riddled with bullet holes and badly damaged, with burst tyres. A lone doll lay abandoned on the promenade where families celebrated the holiday just hours earlier.
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Police are seen trying to halt the vehicle before it starts accelerating towards the Bastille Day crowd in Nice, France.
Tour company Contiki issued a statement confirming all passengers and staff are accounted for and safe.
"Contiki is deeply saddened by the tragic events which have occurred in Nice and our thoughts go out to all those who have been affected," the statement said.
"We can confirm that we have six groups staying in the south of France, four of whom are staying in Nice town, and we can report all passengers and staff have been accounted for and are safe."
Melbourne parents Denae and Sasha Goldsmith are staying in an apartment above where the Bastille Day celebrations were taking place with their two daughters, aged five and seven.
They said they heard the celebration’s music suddenly stop, and looked down at the street from their balcony to see hundreds of people “scrambling” down the street.
“There was a whole lot of noise and then I thought ‘why is the truck there’- it was just coming through the crowd,” Mrs Goldsmith told 9NEWS.
“I thought ‘s--t everyone’s running’ so we grabbed the kids and got down and then the gun shots all happened.”
Mrs Goldsmith said she didn’t see the gunfire because “we got down on the ground”.
The parents told their daughters the sounds were fireworks.
“We put them to bed, so they haven’t seen any of this, thank god,” Mrs Goldsmith said.
From their balcony the couple watched as authorities entered the truck.
“You can see all the gunshots in the front of the truck, where they’ve killed the driver,” Mrs Goldsmith said.
Mrs Goldsmith said there were "hundreds" of police on the scene, and she saw one officer become physically ill as they inspected the inside of the truck.
Tamworth father Dean Ellison said his daughter Hannah, in Nice on a Top Deck tour, phoned him terrified at 6am.
“She said there were bodies and people everywhere. She said it was total chaos,” Mr Ellison told 9news.com.au.
“They were just coming back and walking along the footpath and they heard a heap of screams and yelling.”
Ms Ellison escaped injury but said one of her friends who was on the tour with her is now in hospital.
“They’ve turned around and from what Hannah said they’ve got out of the way just in time.
“She said she knew something was not right and turned around and saw the truck and tried to get out of the way the best she could.
“One of her friends got hurt, but she said she wasn’t serious, but they took her to hospital just as a precaution.”
Mr Ellison said his daughter was in shock and the tour company was working to relocate the group as soon as possible.
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