LE HAVRE: With union activists disrupting fuel supplies, trains and nuclear plants, French Prime Minister Manuel Valls opened the door Thursday to possible changes in a labour bill that has sparked intensifying strikes and protests — but insisted the government will not abandon it.

The reform, aimed at boosting hiring by loosening protection for French workers, has escalated into the toughest challenge yet to President Francois Hollande and his Socialist government.

"There could be improvements and modifications" in the bill, Valls said on BFM television Thursday. He didn't elaborate on what might be changed, and insisted that the "heart" of the bill should remain. Withdrawing the bill "is not possible," he said.

Members of the CGT union immediately dismissed the gesture, saying they want the bill scrapped, not modified. "It's inadmissible," said Arnaud Pacot of the CGT in the Aube region of eastern France said on BFM from a nuclear plant being blocked by activists.

Union activists blocked a major bridge across the Seine River on France's northern coast and a tunnel in Marseille as part of protest actions and one-day strikes around the country Thursday against the bill.

Demonstrators gathered early Thursday morning at a central square in the venerable port town of Harfleur, setting off fireworks and air horns. Activists were unapologetic about the disruption they planned.

"We have to hit where it hurts," said union official Gilles Guyomard. "And where it hurts is the bosses' wallets."

The activists then went to the 2-kilometre-long Normandy Bridge at Le Havre, setting a pile of tires aflame and blocking toll booths. Protesters dispersed two hours later after closures that snarled traffic and stranded motorists.

At least one pair of travelers said they didn't mind.

"It's for us that they're doing this," said Jean-Luc Geraert, whose battered white van was caught behind the makeshift barricade set up at the entrance to the bridge.

Geraert, a 55-year-old industrial painter, said Valls needed to withdraw the law.

"If he doesn't, it's going to get worse."

Valls insisted the bill is "good for workers" and small businesses, and argued that many of its critics are ill-informed of its contents.

The bill loosens the 35-hour workweek, makes it easier to fire workers in times of economic downturn, and weakens the power of unions to set working conditions across an entire sector.