Gangs of masked youths have smashed windows and hurled stones at riot police as thousands of people marched through Paris in protest at a planned change of labour laws that would make hiring and firing easier.

Police fired dozens of volleys of teargas and used water cannon to disperse highly mobile groups of mostly black-clad youths on Tuesday.

The police department reported 13 arrests in the early stages of a street march led by labour unions.

After violent clashes between riot police and masked youths during previous demonstrations, Paris police had feared further incidents and banned 130 would-be troublemakers before Tuesday's rally even began.

It was not immediately clear which group, if any, the youths belonged to.

The government and police have condemned what they say are groups of ultra-violent youths who join protests looking for a fight.

Tuesday's march comes at a time when police are stretched to ensure security during the month-long Euro soccer tournament, with France on maximum alert since Islamist militants killed 130 people in November.

The CGT labour union said the march would be the biggest show of strength since protests over the planned labour reform began in early March.

"This is not the end," CGT leader Philippe Martinez said.

"The struggle is far from over."

The CGT, backed by smaller unions in a campaign of strikes and protests, is sparring for pole position with another big union that backs the reform that would also devolve setting of pay and working conditions more extensively to company level.

In tandem with Tuesday's protest, workers stopped work at the state-owned SNCF rail company, which nevertheless said disruption was far less than at the outset of a rolling strike two weeks ago or on previous occasions this year.