World leaders including David Cameron and Barack Obama have expressed their shock and outrage at the atrocities in Paris.
The Prime Minister vowed the UK will do “whatever we can to help” following the attacks.
US president Mr Obama said the violence in the French capital was “was an attack on all of humanity”.
German chancellor Angela Merkel is “deeply shocked” by the attacks, and has conveyed her sympathy and solidarity, the foreign ministry in Berlin said.
Mr Cameron said: “I am shocked by events in Paris tonight.
“Our thoughts and prayers are with the French people. We will do whatever we can to help.”
The US president told a press conference: “Those who think they can terrorise the people of France and the values they stand for are wrong.”
The UK’s Ambassador to France, Peter Ricketts, said: “We are fully mobilised in the Embassy dealing with appalling events in Paris.
“Solidarity with France.”
Labour leader Jeremy Corbyn added his condemnation of the “heinous and immoral” attacks.
“My thoughts are with the people of Paris tonight,” the Labour leader said.
“We stand in solidarity with the French.
“Such acts are heinous and immoral.”
London mayor Boris Johnson said: “Saddened to hear the terrible news from Paris - my thoughts and those of Londoners are with Parisians tonight.”
Archbishop of Canterbury Most Rev Justin Welby said: “Tragic Paris, desperate news of deep tragedy, with heartbreak for so many. We weep with those affected, pray for deliverance and justice.”
A spokesman for United Nations Secretary-General Ban Ki-Moon said: “The Secretary-General extends his deepest condolences to the families of the victims and wishes a speedy recovery to those injured. He stands with the government and people of France.”
Intelligence experts gave their views on the motives behind the massacre.
John Cohen, a former US Homeland Security Department counter-terrorism co-ordinator, said the presence of multiple attack scenes at the same time suggested a co-ordinated effort to “send a message” and raises immediate terror concerns, including for other cities in Europe and potentially the United States as well.
He said both al Qaida and Islamic State have relied on the strategy of co-ordinated attacks in the past.
Former CIA director R James Woolsey said France was being punished for supporting the fight against extremism.
US Secretary of State John Kerry said the attacks were “heinous, evil” and “vile”, while vice president Joe Biden said: “Such savagery can never threaten who we are.”
The spire of One World Trade Centre in New York will be lit up in red, white and blue in honour of those who died in Paris, the city’s governor Andrew Cuomo said.
Foreign Secretary Philip Hammond said he had spoken to his French counterpart Laurent Fabius about the attack.
Scotland’s First Minister Nicola Sturgeon told BBC Radio Scotland: “This is an unspeakably awful event.
“I think everybody waking up this morning will be deeply shocked, and obviously our thoughts, our prayers, our solidarity, are with the people of Paris and indeed the people of France today.”
“Mayor of London Boris Johnson said: “I have written this morning to Mayor Hidalgo to convey the sense of shock and grief of millions of Londoners at the tragic events in Paris.
“Our thoughts today are with the victims and their families and with the whole French people.
“We regard Paris as our sister city, a place for which we have deep reserves of love and admiration and respect. We are two capitals united in our values - democratic freedom, openness and tolerance.
“The people who launched the attacks last night have no such values. They wish to undermine the things we hold most precious. They want to set neighbour against neighbour. They want to spread fear.
“They will not succeed. As president Hollande has rightly said, they must and will be defeated. We stand ready in London to do everything in our power - to do whatever it takes - to assist in a struggle that embraces us all.”
Pope Francis added his condemnation, telling the Catholic television channel TV2000 the attacks were “not human” and “there is no religious or human justification for it”.