Thousands of people around France have protested against a proposed bill that could make it more difficult for witnesses to film police officers.
Tensions rose at the Paris march as intruders set fire to several cars, broke into a bank and tossed objects at police.
The protest-crashers, who disrupted a similar demonstration a week ago, formed a barricade on a section of the march route in eastern Paris, temporarily blocking the procession.
Dressed in black and known as “black blocs”, the aggressive interlopers are a feared element at French demonstrations.
Police officers who have come under fire for alleged racism and gratuitous violence within their ranks were hard-pressed to stop the individuals seeding chaos at the march.
“Police mutilate, police kill”, read one banner carried by protesters.
Earlier, French president Emmanuel Macron triggered anger among police unions during an interview with young people in which he said that officers with violent behaviour and “racist attitudes” must be tracked and sanctioned.
Mr Macron also announced plans for a online platform the public can use to report and discuss misconduct and acts of bias by police officers.
In reaction, police unions pressed colleagues to stop carrying out identity checks, which activists see as prime opportunities for discrimination.
The interior ministry said about 52,350 people demonstrated around France, including 5,000 in Paris.
Interior minister Gerald Darmanin said at least 64 people were arrested during the protests and eight police officers were injured.
In a tweet, he praised police for facing down “very violent individuals”.
In the western city of Nantes, two riot police were injured, one of them with a Molotov cocktail, French media reported.
At the Paris march, the people who broke into the bank along the route tossed furniture into the street and set it on fire as papers went flying.
Shop windows were broken, and several cars and at least one motorcycle were also set on fire.
Security forces dodged flying objects or stopped them with their shields. One officer was seen blocking a flaming projectile that appeared to be a Molotov cocktail.
“This is urban violence, not a demonstration,” Bruno Bartocetti, head of the SGP-Police FO union, told the CNews TV channel.
Saturday’s marches were focused on a security bill that includes an article aimed at outlawing the publication of images of police officers with intent to cause them harm.
The provision caused such an uproar that the government decided earlier this week to rewrite it.
Critics fear that in its current form, the law could impinge on press freedom and make it more difficult to expose police brutality.
Police misconduct has received fresh attention in France after video footage emerged last month of officers beating up a black music producer, Michel Zecler.
Four of the officers were placed under investigation and two of them were jailed.