PARIS (REUTERS) - Riot police fired teargas and water cannon at protesters marching on Thursday (Sept 15) in France against labour reforms in what unions say will likely be the last demonstrations to try to overturn the law.

Scuffles broke out in Paris and the western city of Nantes. Hooded youths hurled bottles, beer cans and on occasion makeshift firebombs on the fringes of marches against the law that will make hiring and firing easier.

As turnout fades after six months of protests, the head of the Force Ouvriere union signalled that the focus of opposition would now shift to legal challenges against the application of the new law, and that street marches were at an end.

"We are lifting our foot off the pedal for now. We are not going to do this every week," Jean-Claude Mailly told reporters at a rally in Paris's Place de la Bastille square.

Seven months from a presidential election, Mailly said that the unions would not let Socialist President Francois Hollande and his government off the hook.

"This law will be the chewing gum that sticks to the soles of the government's shoes," he told France 2 public television.

Mailly and Philippe Martinez, head of the CGT union, said they hoped legal challenges would force the withdrawal of the new law. They intend to challenge application decrees that will spell out exactly how the law applies on the ground.

The new law, forced through parliament in July, is designed to make France's protective labour laws more flexible, in part by allowing firms to tailor pay and work terms to their needs more easily.

Martinez said the law could be exploited by employers to trim overtime pay from a 25 per cent markup to 10 per cent.

At their peak, the street protests brought close to 400,000 people into the streets last March but turnout has waned over time and was in the low thousands in most cities on Thursday according to early readouts from police.

Police said between 12,500 and 13,500 marched in Paris. More than a dozen people were arrested. Police representatives said about five police were injured.

The government hopes the law will help lower a jobless rate stuck close to 10 percent.

Unions say it will undermine high standards of labour protection as well as their ability to represent workers, notably in small firms where it will give employers more muscle to strike lower-standard deals on issues such as overtime pay.