The Islamic State terror group has claimed responsibility for a string of bomb and gun attacks in Paris which killed at least 128 people and wounded 80 or more.
French president Francois Hollande has denounced the attacks as an "act of war" and vowed that France would be "pitiless" in its response to the "absolute barbarity".
Speaking after an emergency meeting of senior government and security officials at the Elysee Palace, Mr Hollande declared three days of national mourning.
Policing was being strengthened at ports and major events in the UK, and Prime Minister David Cameron was chairing a meeting in Downing Street of the Government's Cobra emergency committee which could raise the official assessment of the threat from international terrorism from its current "severe" level.
A manhunt is under way for 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 televised address to the nation, Mr Hollande said the attacks were "committed by a terrorist army, the Islamic State group, a jihadist army, against France, against the values that we defend everywhere in the world, against what we are: a free country that means something to the whole planet".
He said the attacks were "prepared, organised and planned from abroad, with complicity from within the country".
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.
In a night of carnage in the French capital:
:: Police stormed the Bataclan concert hall where hostages were being held, but attackers wearing suicide belts blew themselves up, leaving 80 people feared dead. A witness said one of the gunmen shouted "Allahu Akbar" and said "This is for Syria"
:: Two suicide attacks and a bombing took place at the Stade de France stadium, where Mr Hollande was among thousands of football fans watching the national side play a friendly against Germany
:: Gunmen targeted bars and restaurants in the 10th and 11th arrondissements of central Paris
:: As many as 18 people died when the terrace of La Belle Equipe was sprayed with gunfire, while around 14 people were killed at Le Carillon bar-cafe. There were also shootings at the nearby Cambodian restaurant Le Petit Cambodge and the La Casa Nostra pizzeria
A message issued in Arabic and French by IS - also known as Isil, Isis and Daesh - said that "eight brothers wearing explosive belts and carrying assault rifles" had targeted "the capital of abominations and perversions and those who carry the crusader banner in Europe".
The locations of the killings were "meticulously selected in advance in the heart of the French capital", said the statement, which described the music fans at the Bataclan as "idolators at a festival of perversity".
In an apparent reference to France's participation in air strikes against IS strongholds in Syria, the statement continued: "France and those who follow the same path must know that they will remain the principal targets of Islamic State and will continue to smell the scent of death for having... struck Muslims in the territory of the caliphate with their aircraft."