Concerns remain about a “small number” of Britons who may have been caught up in the Paris attacks, Downing Street said after David Cameron warned about the possibility of British casualties.
Officials have confirmed the safety of many British citizens in Paris, but a team from the Metropolitan Police and extra consular staff have been sent to work with the French authorities to establish the fate of other UK nationals in the French capital.
The Prime Minister warned that the UK should “be prepared for a number of British casualties” as he told the French people: “Your fight is our fight.”
Islamic State (IS) has claimed responsibility for the attacks which killed at least 127 people and wounded 300 more, around 80 of them critically.
Mr Cameron chaired a meeting of the Government’s emergency committee and also spoke to French president Francois Hollande following the atrocities last night.
A Downing Street spokeswoman said: “ The Foreign and Commonwealth Office have handled around 400 calls overnight.
“While we have now confirmed the safety of many British citizens, we are concerned that a small number of British nationals have been caught up in the attack.
“Our Embassy in Paris are working urgently with the French authorities to find out more and we have deployed additional consular staff and a team from the Metropolitan Police to assist them with this task.”
Mr Cameron told Mr Hollande “that the UK stood with France and the French people during this difficult time and we would do all we can to help”.
The two leaders agreed to “further enhance” co-operation and information-sharing between the UK and France “to ensure we are doing all we can to identify and stop those who threaten us, whether in Syria and Iraq or closer to home”.
In a sign of solidarity with the French, the Tricolore was flown at half-mast alongside the Union Flag over Downing Street.
Mr Cameron said the terror threat level in the UK would remain at “severe” but the Paris attack would prompt a review of plans and suggested the threat posed by Islamic State was “evolving”.
The Prime Minister said: “The events in Paris are the worst acts of violence in France since the Second World War, the worst terrorist attack in Europe for a decade, a horrifying and sickening attack.
“Our hearts go out to the French people and to all those who lost loved ones.
“Today the British and French peoples stand together as we have so often before in our history when confronted by evil.”
He added: “These were innocent victims enjoying a Friday night out with friends and family, no doubt at the end of a hard week. They were not seeking to harm anyone, they were simply going about their way of life - our way of life.
“They were killed and injured by brutal and callous murderers who want to destroy everything our two countries stand for: peace, tolerance, liberty. But we will not let them.
“We will redouble our efforts to wipe out this poisonous, extremist ideology.”
The Queen sent her condolences to the French people in a message to the president, telling Mr Hollande that she and the Duke of Edinburgh were “deeply shocked and saddened by the terrible loss of life”.
A manhunt is under way for possible accomplices of gunmen who targeted a concert hall and the French national football stadium, and sprayed the terraces of bars and restaurants with gunfire in at least six almost simultaneous attacks.
In a message issued in French and Arabic, IS - also known as Isis, Isil and Daesh - said it had targeted “the capital of abominations and perversions and those who carry the crusader banner in Europe”.
In an apparent reference to France’s participation in airstrikes against IS strongholds in Syria, the statement warned the country, and others taking similar action, would “remain the principal targets of Islamic State ... for having struck Muslims in the territory of the caliphate with their aircraft”.
Mr Hollande denounced the attacks as an “act of war” and vowed that France would be “merciless” in its response to the “absolute barbarity”.
The French army and security forces were mobilised “at the highest possible level” and France would “act by all means anywhere, inside or outside the country” against the terrorists responsible, he said.
Speaking after an emergency meeting of senior government and security officials at the Elysee Palace, Mr Hollande declared three days of national mourning.