Pig heads have been found outside several mosques in Paris.
Authorities have decried the insult to Muslims, for whom pigs are considered impure.
More than six million followers of Islam live in France – the biggest number in any country in Europe.
Paris police chief Laurent Nunez said on X that an inquiry had been opened.
“Everything is being done to find the perpetrators of these despicable acts.”
The Paris prosecutor's office said pig heads had been found in front of at least two mosques in Paris and one just outside the city limits.
A pig’s head was also found in a suitcase outside a mosque in the city’s north.
The word "Macron" was scrawled in blue at one of the sites, the prosecutor’s office said.
That was an apparent reference to French President Emmanuel Macron, who is engulfed in a political and fiscal crisis.
A Paris police unit is investigating the incident for suspected incitement to hatred, aggravated by discrimination.
Interior Minister Bruno Retailleau condemned the acts.
“I want our Muslim compatriots to be able to practice their faith in peace,” he said.
“I understand that they feel hurt.”
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