At least six demonstrators and 20 policemen were injured as several hundred clashed with police in Paris on Tuesday in the latest demonstration against disputed labor reforms in France.
The clashes erupted as the international spotlight was turned on France as the host of the Euro 2016 football championships, which have also been marred by violence between fans.
Strikes closed the Eiffel Tower and disrupted transport links as tens of thousands of fans pour into the country for Europe's showcase football event.
France has mustered up to 90,000 police and private guards to provide security for the month-long tournament.
Authorities made six arrests and at least two people were hurt in the Paris unrest, according to an AFP reporter at the scene, as demonstrators stormed a building site and began to hurl wooden palettes at riot police.
Pictures showed one man being led away by officers in riot gear with blood streaming from a wound above his eye, his white T-shirt splattered with blood.
With France on high alert due to the threat of terrorism during Euro 2016, overstretched security authorities feared the demonstrations could turn violent and have banned 130 known troublemakers from taking part.
The terrorism threat was thrust back into the spotlight after a man claiming allegiance to the Islamic State group killed a policeman and his partner at their home in a northwestern Paris suburb late Monday.
The latest in a wave of protests that began in March coincides with a French Senate debate on the reforms, which are aimed at making the job market more flexible and reducing high unemployment but which critics see as too pro-business.
President Francois Hollande's government has voiced hope the latest day of protest will be a last stand for the movement.
But Philippe Martinez, head of the CGT union that spearheaded last month's blockades of fuel depots and an ongoing rail strike, predicted a "very strong mobilization."
The CGT laid on more than 600 buses to transport demonstrators to Paris and said it hoped to attract more support than in March, when it claimed 1.2 million people took to the streets.