Politicians, intellectuals, secular Muslims and left-wing pressure groups have lined up behind Charlie Hebdo, arguing that Muslim groups have no right to call for limits on free speech.
"I just cannot imagine the consequences not only for France but for Denmark and Europe if they lose the case," Fleming Rose, the Danish editor who first published the cartoons, told a news conference with Charlie Hebdo publisher Philippe Val.
"It would turn back the clock decades, ages."
The cartoons, originally published in 2005 in the Danish daily Jyllens-Posten, led to protests in the Muslim world that left 50 people dead.
Several European publications reprinted them as an affirmation of the right to free speech.
In an act of solidarity with Charlie Hebdo, French newspaper Liberation printed the contested cartoons once more on Wednesday.
"It is not words which wound, or pictures that kill. It is bombs," the daily said, calling the trial "idiotic".
A televised debate between Charlie Hebdo publisher Philippe Val and Dalil Boubakeur, rector of the Paris Grand Mosque, broke up acrimoniously on Tuesday after they squabbled over the limits of free speech.
"If we can't criticise religion anymore, there will be no women's rights, no birth control and no gay rights," Val said in the raucous TV debate.
Boubakeur said the controversial cartoon showing Prophet Muhammad with a bomb in his turban was not simply satire, but an insult against all Muslims by suggesting they were all terrorists.
"We don't want censorship, we don't want the sacred to be protected by blasphemy laws or medieval jurisdiction," he said.
Boubakeur said last week he wanted to show that reprinting the cartoons was a provocation equal to acts of anti-Semitism or Holocaust denial, which are both banned under French law.
Separation of religon and politics
An opinion poll released on Tuesday showed 79 per cent of French people thought it unacceptable to ridicule a religion publicly and 78 per cent ruled out parodies of Jesus Christ, Prophet Muhammad or Buddha.