A manhunt is under way for accomplices of gunmen who killed at least 128 people in Paris and wounded many more, as French president Francois Hollande pledged to wage a "merciless" fight against terrorism.
Policing was being strengthened at ports and major events in the UK, and Prime Minister David Cameron was due to chair a meeting of the Government's Cobra emergency committee which could raise the official assessment of the threat from international terrorism from its current "severe" level.
Authorities in France believe that all eight of the attackers responsible for the country's worst night of violence since the Second World War are now dead, including seven who blew themselves up with suicide bombs.
But the city's prosecutor said it is possible there are still other terrorists on the run.
A state of emergency was declared in France after the attacks, which Mr Hollande described as an "abomination".
Police leave was cancelled and some 1,500 extra soldiers have been mobilised to guard official buildings and religious sites, while controls have been re-imposed at the country's borders.
The country's schools and universities, which often open on Saturdays, have been ordered to remain closed.
Mr Hollande, who has cancelled a planned trip to Turkey for the G20 summit this weekend, called an emergency meeting of senior government and security figures at the Elysee Palace.
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 that one of the gunmen shouted Islamist slogans and said "This is for Syria" - a possible reference to France's participation in airstrikes against Islamic State.
:: 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 fixture 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 the La Belle Equipe was sprayed with gunfire, while around 14 people were killed at Le Carillon bar-cafe, and there was shooting at the nearby Cambodian restaurant Le Petit Cambodge.
During a visit to the Bataclan concert hall in the early hours Mr Hollande said the country will be "merciless" against those who have attacked them.
"We will lead the fight. We will be merciless," said the president.
Messages of sympathy and support were issued by world leaders, while ordinary people around the globe turned to social media to express their shock.
Mr Cameron described the attacks as "horrifying and sickening" and vowed the UK will do "whatever we can to help".
US president Barack Obama said the violence in the French capital was "was an attack on all of humanity", while the Vatican condemned it "in the most radical way".
The foreign ministry of Iran - whose president Hassan Rouhani cancelled a trip to France - said that the Paris killers "are not loyal to any type of divine religions -- including Islam".
The Prince of Wales is to send Mr Hollande a message of "profound sympathy and solidarity with the people of Paris", a Clarence House spokeswoman said
The Foreign Office said it was in "close touch" with the French authorities and it was "urgently investigating" whether there were any British victims.
British police were liaising with their counterparts in Paris on possible lessons for security in the UK.
Metropolitan Police assistant commissioner Mark Rowley, the national police lead for counter-terrorism, said there would be a strengthened police presence at ports and major events and on the streets, and called for "vigilance" from the general public.