Fires broke out on the streets of Paris in angry clashes over pension reforms leading to reports of dozens of police injured around the country.
Demonstrators showed their anger at French president Emmanuel Macron's unpopular pension policy on May Day.
French police charged at radical protesters and troublemakers smashing bank and shop windows and setting fires as unions pushed the president to scrap a higher retirement age.
Bank windows and bus stops were smashed, bicycles were torched and cops were also hit by objects after Macron raised the retirement age from 62 to 64 last month.
Footage has also shown policeman lying injured with a road on fire in the French capital.
French police deployed drones to film unrest. A Paris police officer was seriously injured by a Molotov cocktail, and 19 others were hospitalized, among 108 officers injured around France, authorities said. It wasn't known how many protesters were potentially injured.
Clashes also marked protests in Lyon and Nantes.
"Violence is increasingly strong in a society that is radicalizing," the interior minister said on BFM-TV news station, blaming the ultra-left. He said some 2,000 radicals were at the Paris march.
Tear gas hung over the end point of the Paris march, Place de la Nation, where a huge black cloud rose high above the trees after radicals set two fuel cans afire outside a building renovation site, police said. The fire that blackened the facade was later relit.
Labour rights are celebrated across the globe on May 1 and campaigners marked the day by hitting the streets demanding higher salaries, reduced working hours and better working conditions.
Around a million marched across France as part of the protest with 500,000 pouring onto the streets of Paris.
But the General Confederation of Labour (France) claim the figure is much higher with 2.3m demonstrators across the country.
Clashes with police were also reported in Lyon and Nantes.
French union members were joined by groups fighting for economic justice, or just expressing anger at what is seen as Macron's out-of-touch, pro-business leadership. Labor activists from abroad were present, among them Hyrwon Chong of the South Korean Metal Workers' Union.
"Today we see rising inequality throughout the world, terrible inflation," she said, adding that Macron's government was trying "to tear down a pillar of the social system which is the pension system."
Kevin Courtney, joint general secretary of the UK's National Education Union, praised French unions as "an inspiration to working people across Europe." Like them, "you don't back down."
People squeezed by inflation and demanding economic justice took to streets across Asia and Europe to mark May Day, in an outpouring of worker discontent from Tokyo to Pakistan to France not seen since before the worldwide Covid lockdowns.