Satirical magazine Charlie Hebdo is burned out by 1am petrol bomb attack on the day 'sharia' edition was to hit news stands

The offices of the French satirical magazine Charlie Hebdo have been burned out in a petrol-bomb attack after a special Arab spring edition entitled "Sharia Hebdo" featured a cartoon of the prophet Muhammad on the cover as "guest editor".

At around 1am the offices were gutted after a petrol bomb was thrown into a window and set light to the computer system, blowing out windows and burning most of the office contents.

The fire happened before the "sharia" edition was to hit news stands on Wednesday morning in what the paper had mockingly called a "celebration" of the victory of the moderate Islamist party An-Nahda in the Tunisian elections and the Libyan transitional executive's statement that Islamic sharia law would be the country's main source of law. On the front page a cartoon prophet Muhammad says: "100 lashes if you don't die of laughter!"

It is not the first time the weekly has published cartoons of Muhammad, depictions of whom are forbidden in Islam. In 2007 a Paris court threw out an attempt by two Muslim organisations to sue the paper for reprinting the prophet Muhammad cartoons that had been published by a Danish newspaper and sparked protests worldwide.

The magazine's editor, known only as Charb, told France Inter radio that for the past few days the paper had received threats on Twitter and Facebook and by post, which had been passed on to police. He said he was sickened and angry that "violence could be used to counter drawings".

He told Agence France Presse he rejected accusations that he was trying to provoke, saying the magazine was "just doing our job as usual".

The Socialist-run Paris City Hall said it would help Charlie Hebdo find a new office. "We'd never leave an organ of the free press without offices. It's the duty of the republic. Charlie Hebdo should be able to continue to work," tweeted Christophe Girard, a deputy mayor of Paris.

Mohammed Moussaoui, head of the French Council for the Muslim Faith (CFCM), said he "strongly condemned" the fire if police did confirm it was a criminal attack.

In 2006, the CFCM brought a lawsuit against Charlie Hebdo for publishing caricatures of the prophet that he said were "a lot more violent". He told AFP the front page of this week's edition "wasn't of the same order" but added that the simple fact of caricaturing the prophet was considered offensive to Muslims.

Jean-Francois Copé, secretary general of Nicolas Sarkozy's ruling rightwing UMP party, condemned the apparent arson attack, telling Europe 1 radio it was "an attack against a newspaper in a country which should stand for freedom of expression".