French president Francois Hollande has denounced the terror attacks that killed at least 127 people in Paris as an "act of war" and blamed the Islamic State terror group for the carnage.
Speaking after an emergency meeting of senior government and security officials at the Elysee Palace, Mr Hollande declared three days of national mourning and vowed that France would be "pitiless" in its response to terrorism.
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.
French authorities said they believed all eight of those involved in the attacks were dead - seven of them killed by suicide bombs - but Paris's chief prosecutor said it was possible other terrorists were still on the run.
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.
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 added that France "will be merciless toward the barbarians of Islamic State group", and "will act by all means anywhere, inside or outside the country".
Mr Hollande said the French army and security forces were mobilised "at the highest possible level" and insisted France would "triumph over barbarity".
"What we are defending is our country, but more than that, it is our values," 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" - a possible reference to France's participation in air strikes 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 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 state of emergency was declared in France after the worst night of violence in the country since the Second World War.
Police leave was cancelled and 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. All public demonstrations were banned in the Paris region until Thursday.
Mr Hollande, who has cancelled a planned visit to Turkey for the G20 summit this weekend, is to address both houses of the French parliament at Versailles on Monday.
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 "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 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 French authorities and was "urgently investigating" whether there were any UK 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 in Britain, and called for "vigilance" from the general public.
Police sources in Paris said around 180 people were injured, including 80 who were in a critical condition.
US officials said all members of the American rock band Eagles of Death Metal, who were playing a concert at the Bataclan, escaped unhurt.
Scenes of "carnage" were described by witnesses in the concert hall, who said there was "blood everywhere".
The brother of the band's drummer, Julian Dorio, told US newspaper the Atlanta Journal Constitution that the musicians hit the floor after seeing gunmen, then fled by a backstage door.
After speaking to his brother by phone, Michael Dorio said: "They saw a man with a machine gun just opening fire."
Television cameraman Charles Pitt said he was outside a cafe in the city's 11th arrondissement where people were shot at around 9.10pm local time.
He told BBC News: "I had literally gone about 30 metres when, I thought it was a firecracker to start with, and then it went on and it got louder.
"It went on for a minute. Everybody dived for cover, thinking it was gunfire. Then there was a pause for about 15 seconds and then it all started up again.
"Then it calmed down a bit and I walked back to the front of the cafe and there was a whole pile of bodies, probably about seven on the left-hand side and four that had been sitting on the tables outside on the right-hand side, and a lot of injured.
"I saw a woman who had obviously been shot in the leg."
The Foreign Office advised Britons to "exercise caution in public places" following the attacks, and people with concerns about British friends or relatives in Paris can call 020 7008 1500 for assistance.
French authorities have advised people in Paris to remain in their homes.
Disneyland Paris was closed for the day and all sporting events in the French capital have been postponed, including a rugby union Champions Cup tie involving Scotland's Glasgow Warriors.
Channel Tunnel train operator Eurostar said services would run to Paris, but passengers due to travel on Saturday were being offered a free ticket exchange.
The attacks come after the Charlie Hebdo atrocity in January, which saw 12 people killed after gunmen stormed the Paris offices of the satirical magazine.
They also came a day after Islamic State militant Mohammed Emwazi, known as Jihadi John, was targeted in a US air strike in Syria. It is not clear if there is any link.