France's hardline CGT union sought to choke off power and fuel supplies and hamper the public transport network on Thursday in a showdown with a government that flatly refused to withdraw a contested labour law reform.

As tens of thousands of demonstrators took to the streets, workers responded to the union call by stopping work at oil refineries, nuclear power plants and the railways, as well as erecting road blocks and burning wooden pallets and tyres at key ports like Le Havre and near key distribution hubs.

The standoff, which raises the spectre of disruption during the France-hosted European soccer tournament that opens on June 10, was condemned both by Socialist Prime Minister Manuel Valls and the country's other big trade union, the CFDT.

After months of rolling protests sparked by a reform that aims to make hiring and firing easier, Thursday's stoppages and street marches were being watched closely as a test of whether the CGT-led opposition is solid or at risk of fizzling out.

The street marches were joined by scores of marchers from a youth protest movement called Nuit Debout (Up All Night). Police deployed to counter risks of the severe fringe violence in which 350 police and several protesters have been hurt and more than 1,300 arrested at similar rallies in recent weeks.

CGT chief Philippe Martinez, asked by Reuters if his union was willing to distrupt the Euro 2016 soccer contest, said: "The government will has the time to say 'let's stop the clock' and everything will be ok."