One French police officer was seriously burnt and two others injured in clashes at a May Day demonstration in Paris on Monday in which protesters threw Molotov cocktails and other missiles, the police said.
Television pictures showed policemen trying to shake flames from their riot gear, and of tear gas enveloping the streets around Paris' Bastille monument.
This year's May Day came less than a week ahead of the final round of a presidential election where voters must choose between the far-right National Front's Marine Le Pen and centrist Emmanuel Macron.
Some trade unionists and left-wing activists sought to make the day one of national solidarity against the National Front, mirroring protests in 2002 when Le Pen's father, party founder Jean-Marie Le Pen, was a candidate.
Marine Le Pen tweeted her support for the injured policemen and said the incident was the type of unacceptable behaviour that she no longer wanted to see on French streets.