Several dozen youth protesters clashed with police in the early hours of Friday in a central Paris square after refusing orders to disperse, police said.

Police detained 24 people overnight after scenes of violence that saw protesters hurling chunks of concrete at riot police and torching vehicles.

Around 100 "especially violent demonstrators" forced their way through a police barricade at the Place de la Republique, police said.

The square has been the venue for the past month of nightly gatherings dubbed "Up All Night", but on Thursday around 1,000 protesters were allowed to stay only until midnight.

The new violence followed clashes on Thursday during protests against France's hotly contested labour laws that left 24 policemen injured, three seriously.

At least 170,000 workers and students had taken to the streets nationwide to press demands for the withdrawal of the proposed labour law.

Opponents of the labour reform, billed as an effort to reduce chronic unemployment, which stands at 10%, say it will threaten cherished rights and deepen job insecurity for young people.

Interior Minister Bernard Cazeneuve visited an officer at a Paris hospital late on Thursday, saying he remained in serious condition.

Police said he was a plainclothes officer who was hit in the head by a projectile.

Speaking to reporters outside the hospital, Cazeneuve praised the "great professionalism" of security forces in bringing the protests under control while "violent groups" sowed disorder.

But he rejected demands for an outright ban on demonstrations from right-wing politicians who argue that France remains under a state of emergency since the November jihadist attacks in Paris.

"Those who try to exploit this period for political ends by suggesting that we ban everything are both ignorant of the law and ignorant of the state of emergency," the interior minister said. Hundreds arrested  

To unions who have accused police of heavy-handedness in quelling protests, Cazeneuve said the charges served only to heighten tensions and did "not show a great spirit of responsibility."

The far left has also denounced "police violence" while the right-wing Republicans party has called for the Up All Night ("Nuit Debout" in French) protests to be banned.

Trade unionist Olivier Besancenot accused the government of inviting unrest through an excessive show of force.

"With this kind of deployment (of security forces) you know exactly what it will incite," he said. "You can create conditions for things to go wrong."

The protests kicked off on March 31 inspired by opposition to the proposed labour reforms, but the movement has since embraced a range of grievances and has spread to several other cities in France.

The event, which has drawn up to 3,000 mainly young people at a time in Paris, has increasingly been marred by violence, with police warning the organisers not to let their peaceful causes be hijacked by troublemakers.

On several occasions, small groups of hooded youths have moved into the square, apparently determined to clash with police.

More than 400 people have been arrested since the demonstrations began.