An international manhunt was continuing for a suspect linked to the Paris attacks as the UK prepared to recruit almost 2,000 more spies to counter the Islamic State (IS) threat.
Police across Europe are searching for SalahAbdeslam, 26, who rented a car used to carry gunmen to the Bataclan music venue in Paris which became the scene of a massacre.
Britons have been urged to join a Europe-wide minute's silence at 11am to remember the 129 people killed in the rampage in the French capital.
David Cameron will join world leaders at the G20 summit in Turkey in observing the silence in remembrance of the victims.
The French authorities missed an opportunity to detain their target Abdeslam just hours after the carnage in Paris when he was questioned and released on Saturday morning.
Officers had Abdeslam in their grasp when they stopped the car carrying him and two other men near the Belgian border.
Abdeslam is one of three brothers suspected of involvement in the Paris attacks, one of whom - reportedly called Ibraham - died in the Bataclan while the other was arrested in Belgium.
IS - also known as Isil and Daesh - has claimed responsibility for the Paris atrocities which killed at least 129 people and French forces struck back with a massive bombardment of the jihadist group's stronghold in Raqqa, Syria.
Twelve aircraft, including 10 fighter jets, dropped 20 bombs, destroying a jihadi training camp and a munitions dump in Raqqa, where Iraqi intelligence officials claimed the attacks on Paris were planned.
British police and spies are working closely with counterparts in France and Belgium to identify and pursue those behind the Paris massacre.
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.
The security and intelligence services will receive a major funding boost in response to the IS threat, which has been blamed for the bombing of a Russian airliner in Egypt and the Paris attack in recent weeks.
Mr Cameron has announced a 15% increase in the 12,700-strong staff of the security and intelligence agencies MI5, MI6 and GCHQ with the recruitment of an additional 1,900 personnel.
Spending on aviation security will increase to at least double the current £9 million a year.
The Prime Minister said the UK was engaged in a "generational struggle" against extremist terror. The additional spending will help "combat those who would destroy us and our values" and allow Britons to "continue with our way of life we hold so dear".
More details have emerged about the IS attack which was the worst terrorist outrage in Europe for more than a decade, leaving at least 129 dead and 350 wounded.
:: Prosecutors believe three teams of terrorists carried out the co-ordinated attacks
:: As many as three of the seven suicide terrorists killed on Friday night were French. Two were Frenchmen living in Brussels
:: Bilal Hadfi has been named in reports as one of the assailants and had is said to have fought with IS in Syria
:: Three Kalashnikov assault rifles were discovered inside the 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
:: Seven people have been arrested in Belgium and six in France in connection with the killings, including Mostefai's father and brother
:: At least one of the men arrested in Belgium was a French national
:: It has been reported that two of the attackers passed through Europe as refugees, one using a Syrian passport to enter Greece and the other claiming asylum in Serbia
The only British fatality confirmed so far is Nick Alexander, 36, from Colchester, who was selling merchandise for rock group Eagles of Death Metal when their gig at the Bataclan 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 as "intensely moving".
In a sign of the continued tension in Paris, the Place de la Republique - where huge crowds had gathered - was suddenly evacuated with people fleeing in terror.
The square, where Channel 4 News was broadcasting live at the time, was reopened after it was confirmed the panic was the result of a false alarm.
Speaking from the G20 summit in Turkey, Mr Cameron said Europe would be safer if the threat from IS - also known as Isil - was dealt with.
He said: "It's become even more clear that our safety and security depends on degrading and ultimately destroying Isil whether it's in Iraq or Syria.
"We're playing a huge role in that already in Iraq. Others are taking action in Syria which we both support and enable, but we've got to keep on making the case that we will be safer in the UK, in France, right across Europe if we destroy this death cult once and for all."
It has emerged Iraqi intelligence warned countries in the US-led coalition against IS, including France, of an imminent assault the day before the Paris attacks.
But the Iraqi dispatch provided no details on when or where the attack would take place, and a senior French security official described it as the kind of warning French intelligence gets "all the time" and "every day".