PARIS (Reuters) - Gangs of masked youths hurled stones and makeshift firebombs at riot police on Tuesday as tens of thousands of people marched through Paris in protest over a planned change of labor laws that would make hiring and firing easier.

Police used dozens of rounds of teargas and water cannon to disperse highly mobile groups of mostly black-clad youths. The Paris police department reported 58 arrests, including many foreigners, with 24 police and 17 protesters injured.

The clashes have piled additional pressure on a police force struggling to maintain security during the month-long Euro soccer tournament. France also remains on maximum alert after Islamist attacks last November killed 130 people. Late on Monday a policeman and his wife were slain at their home by what authorities said was an Islamist-inspired man wielding a knife.

The hardline CGT labor union had called for a large turnout in Tuesday’s protest march to demonstrate that opposition to the labor reform remains strong despite waning turnout at rallies and strikes. Its call appeared to have been heeded.

Police put the turnout at 75,000-80,000 in Paris alone, roughly three times more than a previous highpoint. Unions, whose figures always substantially exceed police estimates, put the tally at up to 1.3 million.

“This is not the end,” CGT leader Philippe Martinez said. “The struggle is far from over.”

Backed by smaller unions in a campaign of strikes and protests, the CGT is sparring for pole position with another large union, the CFDT, which backs a reform which would also further devolve the setting of pay and working conditions to company level.

The police department condemned the fresh spate of violence, and Interior Minister Bernard Cazeneuve denounced rioters who among other things, he said, vandalized a hospital which was tending to the three-year-old child of the slain policeman.

On many previous occasions, gangs of what the government and police have described as ultra-violent youths have engaged in running battles with law enforcement officers, ransacking shoptfronts and street furniture.

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

Ninety percent of high-speed connections were operating and other services were working at about 70 percent, the SNCF said.

President Francois Hollande’s Socialist government has said it will not withdraw the labor reform, though it has already been watered down. It forced it through the lower house of parliament by decree last month and aims to make it law by July.

The CGT and the smaller Force Ouvriere as well as student unions say the reform will lower standards of labor protection.

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The government and the large CFDT union argue the contrary, saying it will help tackle a jobless rate of 10 percent and also develop labor representation at grassroots level.