Tensions erupted at a May Day rally in Paris on Sunday as thousands of protesters used traditional Labour Day marches to show newly re-elected President Emmanuel Macron the opposition he could face in his second five-year term.
Demonstrators broke windows at some banks and a fast-food restaurant, ripped up street signs and burned barricades in the streets - apparently the work of men dressed and masked in black.
Police moved in, firing rounds of tear gas.
May Day is often a time of high emotion for trade unionists and other workers, and protests in the last two years have been limited by pandemic restrictions.
Some 250 marches and protests were being held around France, with the Communist-backed CGT union leading the main march through eastern Paris.
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All were pressing Macron for policies that put people first and condemning his plan to raise France's retirement age from 62 to 65.
Macron says that is the only way the government can continue to provide good retirement benefits to the French.