British police and spies are assisting the international manhunt for terrorists involved in planning the Paris atrocities.
Security has been beefed up in UK cities and ports as Britons were urged to remain vigilant, although the terror threat level has not been changed from the second-highest "severe" rating.
Speaking after leading a meeting of the Government's Cobra emergency committee, Home Secretary Theresa May said the UK authorities were working to help find anyone involved in the "barbaric attacks" in the French capital.
Her comments came as more details emerged about the Islamic State (IS) attackers who carried out the worst terrorist outrage in Europe for more than a decade.
:: Seven people were detained in Belgium in connection with the attacks.
:: Two of the terrorists killed on Friday night were Frenchmen living in the Belgian capital Brussels.
:: Three Kalashnikov assault rifles were discovered inside a Seat car used in the attacks which was found in the suburb of Montreuil, four miles east of Paris.
:: One of the attackers was identified as 29-year-old Frenchman Ismael Mostefai, who had been flagged for links to Islamic radicalism
:: Mostefai's father and brother have been arrested.
:: Prosecutors believe three teams of terrorists carried out the co-ordinated attacks.
:: At least one of the attackers is believed to have passed through Greece as a refugee.
At least 129 people were killed during the carnage in Paris on Friday night, with a "handful" of Britons feared to have died.
Nick Alexander, 36, from Colchester, the only confirmed British fatality, was selling merchandise for rock group Eagles of Death Metal when their concert at the Bataclan venue was targeted.
Video has emerged of the moment the terrorists attacked, firing repeatedly at fans as band members fled the stage.
The UK's ambassador in Paris, Peter Ricketts, laid flowers at an impromptu shrine outside the venue today, describing it "intensely moving".
Setting out the British response, Mrs May said: "The UK police and security services are working very closely with their counterparts in France and Belgium to identify all those involved and to pursue anyone who may have been involved in the preparation of these barbaric attacks."
She added: "The UK stands shoulder to shoulder in solidarity with France. The terrorists will not win, we will defeat them."
Extra security measures have been put in place around Britain, Mrs May said.
"People will see increased security at the borders, increased checks taking place.
"There is also some increased police presence in major cities as well."
Mrs May confirmed that there were "tried and tested" measures for the military to respond to a marauding attack by terrorist gunmen.
Reports have suggested that special forces are geared to intervene if an attack like the one carried out in Paris was attempted on British soil.
The Home Secretary said: "Since the firearms attack that took place in 2008 in Mumbai, we have been building the capability of police here in the UK to respond to a multiple firearms attack and developing that capability - different training for the police and ensuring that they are able to respond quickly to such an event.
"Indeed there was a major exercise just this summer on the streets of London testing what their response would be to an attack of this sort.
"Of course there are tried and tested arrangements in place for military support to be provided to the police when that's necessary.
"We are obviously reviewing these arrangements to see whether there is anything we need to learn from what has happened in Paris to further develop our capability."
A team from the Foreign Office has travelled to Paris to assist Britons injured or traumatised by the violence.
Mrs May said: "There is considerable consular work being done by the British embassy but also a crisis team was sent out from the Foreign Office here in the UK to provide extra support to those who are in hospital, those who have obviously been caught up in this.
"Support will also be available to those who are returning who have been part of witnessing these terrible events."