Riot police on Thursday arrested 16 people in central Paris in an attempt to quell a violent nationwide protest against a controversial labor law reform.

Earlier in the day, police deployed tear gas after thousands of dock workers poured into a public square in the French port city of Le Havre, setting off smoke bombs.

A police spokesman estimated that between 18,000 and 19,000 people were taking part in Thursday's protest.

As union activists disrupted fuel supplies, trains and nuclear plants during a day of demonstrations around the country, French Prime Minister Manuel Valls opened the door to possible changes but said his government would not abandon the bill, which would make the country's 35-hour work week more flexible, among other provisions.

The draft law, aimed at boosting hiring after a decade of nearly 10 percent unemployment and slow but corrosive economic decline, 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 — a measure weakening the power of unions over workplace rules — should remain.

Withdrawing the bill "is not possible," he said.

Members of the CGT union, leading the protests, said it was too late to compromise. Many remained angry that the government forced the bill through the lower house of parliament without a vote because of division in the Socialist majority.

"Valls is hardening his tone? Well were hardening our tone too!" an organizer shouted into a loudspeaker at the Normandy Bridge, where some 200 to 300 trade unionists and other protesters gathered to block traffic in one of many disruptions to roads around the country Thursday.

After clearing the bridge, which typically carries about 12 million vehicles a year, union activists spread the disruption to Le Havre, driving slowly through town or briefly blocking roads on foot as cars around them honked repeatedly.

Then the dock workers stormed the main square in front of City Hall, setting off multicolor smoke bombs and throwing some in fountains, kicking up plumes of water. The sounds of sirens and the smoke bomb explosions reverberated around the area.

Fabien Gloaguen, an activist with the militant Worker's Forcemovement, said the government would have to back down. "He's going to withdraw it," Gloaguen said.

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

In addition to loosening rules about the 35-hour work week, the bill 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.

Two months of protests escalated over the past week as unions targeted the sensitive oil industry, blocking fuel depots and refineries.

The government has started using its strategic fuel reserves and forcing depots to reopen, but supplies remained spotty Thursday, with long lines and caps on purchases.

Drivers endured long waits to reach gas pumps, railing at the strikes, the government and the overall funk in France.

Gas station manager Bernard Ballaux has limited customers to $56 worth of diesel after a week of panic buying. Customers "fear that they won't have gas anymore so they are irritated, tense."

Prices have risen noticeably at gas pumps.

But at the blocked Normandy Bridge, 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. Geraert, a 55-year-old industrial painter, said if the government doesn't back down soon, "it's going to get worse."

The Associated Press contributed to this report.