The headquarters of French satirical magazine Charlie Hebdo were gutted on Wednesday by what its editor said was a firebomb, after it put an image of the Prophet Mohammed on its cover.
This week's edition shows a cartoon of Mohammed and a speech bubble with the words: "100 lashes if you don't die of laughter". It has the headline "Charia Hebdo", in a reference to Muslim sharia law, and says Mohammed guest-edited the issue.
Charlie Hebdo's website on Wednesday appeared to have been hacked and showed images of a mosque with the message "no god but allah".
Many Muslims find any depiction of Mohammed offensive. Danish cartoons in 2005 sparked protests in the Muslim world in which at least 50 people were killed. Protests in April against an American pastor's burning of the Koran led to unrest in Afghanistan in which several died.
A police source stopped short of blaming the blaze at the Paris magazine offices on a firebomb and said it happened around 1 a.m. (midnight GMT), adding that no one had been injured.
"The building is still standing. The problem is there's nothing left inside," Stephane Charbonnier, editor of Charlie Hebdo, told Europe 1 radio.
The magazine had received many emails containing insults and threats in the past few days, Charbonnier said.
"It's clear that it's impossible to put together a paper in these conditions. For next week we will find offices elsewhere," the editor said. "In any case there is no question that we will give ground to the Islamists. We will continue."
Freedom of expression
Prime Minister Francois Fillon said he had asked Interior Minister Claude Gueant to ensure the perpetrators of the attack were tracked down.
"Freedom of expression is an inalienable value of democracy and any incursion against press freedom must be condemned with the utmost force. No cause justified violent action," he said in a statement.
Tareq Oubrou, head of the Association of Imams of France, also condemned the attack. "This is an inadmissible act," he told French TV station i>tele.
"Freedom is very important. It is nonetheless important to underline the sensitivity of the situation we face today.
"I call on Muslims to treat this lucidly and not succumb to what they consider as provocations regarding their religion ... I personally call on Muslims to keep an open mind and not take this too seriously."
The editor of the left-wing daily Liberation offered to house Charlie Hebdo staff and to publish work its cartoonists had prepared ahead of this week's meeting of G20 leaders in Cannes, southern France.
Television footage of Charlie Hebdo's headquarters showed trash strewn on the sidewalk and a burnt-out interior.