The French authorities have launched inspections of dozens of mosques and prayer halls suspected of links to Islamist extremism.
Interior Minister Gérald Darmanin announced the crackdown, saying some could be closed if found to be encouraging “separatism”.
It comes a week before the unveiling of a new law to combat such extremism.
It is a response to attacks in October, blamed on Islamists, including the beheading of teacher Samuel Paty.
In a note to regional security chiefs, reported by French media, Mr Darmanin said there would be special checks and surveillance for 76 mosques and prayer halls, 16 of them in the Paris region.
He ordered “immediate action” concerning 18 of them, with the first checks set to be done on Thursday.
In a tweet he called it a “massive and unprecedented action to combat separatism”.
State inspectors will examine their financing, their imams’ connections and possible Koranic schools for young children, Reuters news agency reports.
The BBC’s Hugh Schofield in Paris says Mr Darmanin’s authority has been dented in recent days by a row over police abuses in France and a proposed new law to protect police officers’ identities.
‘Not widespread radicalisation’
The 76 addresses set to be visited by inspectors are only a fraction of about 2,600 Muslim places of worship in France.
Mr Darmanin said that fact showed that “we are far from a situation of widespread radicalisation” among Muslims in France.
“Nearly all Muslims in France respect the laws of the republic and are hurt by that [radicalisation],” he said.
France was shocked by the beheading of Samuel Paty and the fatal stabbing of three people in a cathedral in Nice.
Mr Paty had been vilified by some on social media after showing controversial Charlie Hebdo cartoons of the Prophet Muhammad to a class.
A top court has ordered the closure for six months of the Great Mosque of Pantin, after it published a video denouncing Mr Paty. The mosque is in a low-income area on the capital’s north-eastern outskirts.
State secularism – or laïcité – is central to France’s national identity. Laïcité decrees that the public space – whether classrooms, workplaces or ministries – should be free of religion.