Francois Fillon, the French prime minister, has criticised the attack on French satirical weekly, Charlie Hebdo, after it "invited" the Prophet Muhammad to be a guest editor and printed an image of him on its cover.

A firebomb attack caused serious damage to the headquarters of the weekly based in French capital, Paris.

"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," Fillon said in statement on Wednesday.

Claude Gueant, the French interior minister, said that everything would be done to find the perpetrators of the attack.

"You like or you don't like Charlie Hebdo but it's a newspaper. Press freedom is sacrosanct for the French," he said.

Police said nobody was injured in the fire that broke out at about 1am local time (midnight GMT) in the office building that houses the weekly.

The Paris prosecutor's office told the Reuters news agency that two molotov cocktails had been thrown into the magazine's offices.

The French Muslim Council (CFCM), a prominant organisation representing Muslim faith in the country, denounced the attack while also faulting the satirical publication.

"The CFCM deplores the deeply mocking tone of the newspaper towards Islam and its prophet, but reaffirms with force its total opposition to any act or form of violence," it said in statement.

No provocation

The weekly had said it would publish a special edition on Wednesday to "celebrate" the Ennahda party's election victory in Tunisia and the transitional Libyan executive's statement that sharia law would be the country's main source of law.

The director of Charlie Hebdo, who uses the name Charb, said on French television that "the material damages are large" and said many computer files were destroyed, as he stood in front of piles of scorched papers and equipment.

Charb rejected accusations that he was trying to provoke Muslims.

"We feel we're just doing our job as usual. The only difference is that this week, Muhammad is on the cover and that's quite rare," he told the AFP news agency.

Stephane Charbonnier, editor of Charlie Hebdo, told Europe 1 radio: "The building is still standing. The problem is there's nothing left inside."

The weekly's website on Wednesday appeared to be offline amid earlier reports that it had been hacked. Reuters reported the site had earlier showed images of a mosque with the message "No God but Allah".

Newspaper employees said they had received many threats as a result of the issue, subtitled "Sharia Hebdo", in reference to Islamic law.

A Paris court in 2007 threw out a suit brought by two Muslim organisations against Charlie Hebdo for reprinting cartoons of Prophet Muhammad that had appeared in a Danish newspaper, sparking angry protests by Muslims worldwide.