Protesters in France have clashed with police against a controversial labour bill that they say is too "pro-business" and would make it easier for companies to lay off staff.

There were sporadic clashes in the capital, Paris, on Thursday as tens of thousands of people took part in street marches against a deeply unpopular reform package.

In the western city of Nantes, police fired tear gas at protesters; while in the northern port of Le Havre, hundreds of strikers, mainly dockworkers, blocked all the main access routes into the city with barricades of burning tyres, the BFMTV rolling news channel reported.

The demonstrations, part of a nationwide strike against changes that could alter France's 35-hour working week, have gripped the country for weeks.

Opponents to the reform, billed as an effort to lower France's stubborn 10 percent unemployment, say it will deepen job insecurity for young people and threaten cherished workers' rights.

The head of France's large General Confederation of Labour (CGT) union attacked the proposed law, saying it would allow employers to short-circuit national regulation of basic worker rights by giving bosses greater freedom to set terms of pay, rest and overtime rates.

"We want it withdrawn as long as the goal means the law is no longer the rule, and that every company can opt out on work time or overtime rates. That's unacceptable," CGT chief Philippe Martinez told the Reuters news agency.

Protests against the reform started on March 9, culminating in massive demonstrations on March 31 that brought 390,000 people on to the streets, according to an official count. Organisers put the number at 1.2 million.

With little more than a year left in his mandate, France's deeply unpopular President Francois Hollande has been banking on the labour reform as a standout initiative with which to defend his record.