A director said a petrol bomb had been thrown inside the paper's offices, but police said the cause was unclear [AFP]

Police say a fire has caused serious damage at the headquarters of a satirical French newspaper that "invited" the Prophet Muhammad to be a guest editor and planned to print a cartoon image of him on the cover.

A police official said the fire broke out overnight on Tuesday at the offices of Charlie Hebdo, and the cause remained unclear. The official spoke on condition of anonymity because an investigation into the fire is under way.

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 the newspaper, 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.

"A window was broken and a Molotov cocktail was thrown inside," Charb said. "There's nothing left inside."

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 AFP.

The magazine'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.