David Cameron has said the West must stop "the poisoning of young minds" by Islamic State and other extremists.
The Prime Minister said he had spoken to Francois Hollande to offer his support after the attack near Lyon and would also speak to the Tunisian government following the attack there to offer "our solidarity in fighting this evil of terrorism".
"This is a threat that faces all of us," Cameron said. "These events have taken place today in Tunisia and in France but they can happen anywhere. We all face this threat."
"The people who do these things, they sometimes claim they do it in the name of Islam. They don't. Islam is a religion of peace," he said.
"They do it in the name of a twisted and perverted ideology that we have to confront with everything that we have."
David Cameron had been due to talk with French President Francois Hollande about the situation in Calais this morning.
ITV News Deputy Political Editor Chris Ship reports:
David Cameron has spoken to French President Francois Hollande to convey his sympathies "over what appears to be an appalling incident", Downing Street said.
"Details are still emerging, so we wait to see those," the spokesman continued. "But it clearly looks an extremely concerning situation and our thoughts are with all those affected by it."
David Cameron has defended the Government's plans to cut a further £12 billion from the welfare bill, saying he was determined to transform Britain into "a higher wage, lower tax, lower welfare society".
Mr Cameron said Britain needed to end the "merry-go-round" where people on low pay paid tax to the Government only for the Government to hand back the money in welfare payments.
"We need to move from a low wage, high tax, high welfare society to a higher wage, lower tax, lower welfare society," the Prime Minister said in a speech in Runcorn in Cheshire.
Mr Cameron's comments come amid speculation ministers are preparing cuts to tax credits in his first all-Conservative Budget next month.
Andy Burnham, the Labour leadership candidate, has said it is "disgraceful" that ministers had yet to outline where the cuts would come.
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Prime Minister David Cameron has said the Magna Carta was "revolutionary" in shaping the world and is "as relevant today" as it was eight centuries ago.
Speaking at a 800th anniversary event at Runnymede Meadows in Surrey where the historic document was sealed, he said the "Magna Carta is something every person in Britain should be proud of".
"On this historic day, let us pledge to keep those principles alight, let us keep Magna Carta alive, because as those barons showed all those years ago, what we do today will shape the world for many, many years to come."
The prime minister has had a dinner-time row with the Argentine foreign minister after he brought up the Falklands over dinner.
Hector Timerman raised the issue of the ownership of the islands during a meal-time meeting between EU leaders and Latin American representatives.
According to Argentinian media, he said: "The Argentine government expects countries from the European Union to support the United States resolution that urges Argentina and the UK to dialogue over the Malvinas Islands."
Mr Cameron demanded to be allowed to respond even though it wasn't his turn to speak, saying "everyone has the right to self-determination".
He also argued that it was "totally inappropriate" for Argentina to be "threatening" investors in the area, according to Downing Street.
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