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.”
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.
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.
He told BBC News: “The fact that France has been a good ally of the US and Britain in this struggle with ISIS to try to hold things down has worked to France’s detriment because it’s a lot easier to get to France from Syria and so forth than it is to get to the United States or even Britain.
“I think there is a very good chance that a flow of refugees, war refugees, whatever you want to characterise them as, out of the Middle East and especially out of Syria has created the background which makes something like what we’re seeing here possible.”
He added: “What’s interesting to me is the multi-point operation here, hostage taking in one place, gun fights in others, executions.
“This is put together not just by a group of crazy, emotional terrorists but by something looking a bit like part of a government that plans things carefully.
“I would think it would probably be ISIS. It wants to be a government.”
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.
US Attorney General Loretta Lynch said America stands in solidarity with France.
“This is a devastating attack on our shared values and we at the Department of Justice will do everything within our power to assist and work in partnership with our French law enforcement colleagues,” she said.