Pig heads appeared outside several mosques around Paris on Tuesday, authorities said, decrying the insult to Muslims.
France has Europe's largest population of Muslims, over 6 million, for whom pigs are considered impure.
"An inquiry has been immediately opened," Paris police chief Laurent Nunez said on X. "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 head was also found in a suitcase outside a mosque in the north of the city, it said.
The word "Macron" was scrawled in blue at one of the sites, it added, 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, calling them "outrageous" and "absolutely unacceptable".
"I want our Muslim compatriots to be able to practice their faith in peace," he said.