Cardinal Jean-Marie Lustiger, archbishop of Paris said bishops around the country had reported dozens of cases in which Roman Catholic girls had been harassed for wearing a cross, or nuns criticised for appearing in public in their habits.
"At a university in Paris, a woman wearing a small cross had it torn off by other students," Lustiger said. "A nun who was crossing a street in the dress of her religious order was told by passers-by: 'You shouldn't go out in your habit'.
"I could tell you dozens of other cases the bishops have reported," he added.
Lustiger blames politicians for stirring up old passions by debating a planned ban on the Muslim veil in public schools.
The law, which a wide spectrum of French politicians and voters support as a bulwark against rising Islamist influence among Muslim immigrants, would be "just the beginning of a long crisis", he told France-Inter radio.
"This law, with all its excesses, risks opening up a war of religions... The politicians should realise what they are stirring up."
Cardinal Jean-Marie Lustiger
Leaders of France's five-million strong Muslim communities have also reported recent cases of harassment, such as banks and municipal offices refusing to serve veiled women, since President Jacques Chirac announced the planned ban last month.
Lustiger said France's strict separation of church and state in 1905 had calmed tensions after the sometimes violent struggle of anti-clerical politicians who curbed the privileges of the once-mighty Catholic Church.
For many French people, that struggle ranks second only to the 1789 revolution as a milestone in the creation of modern Frace - one reason why so many refuse to make concessions now to Islam.
Lustiger stressed France had "religious peace" now but added: "This law, with all its excesses, risks opening up a war of religions... The politicians should realise what they are stirring up."
The cardinal, the son of Polish Jewish immigrants and a teenage convert to Catholicism, said the Church accepted the secular system and would not oppose a law once passed.
But he doubted it would be effective, predicting instead an endless series of trials.