By Silvia Aloisi

PARIS (Reuters) - France braced for a new round of nationwide protests and strikes on Thursday after a meeting between the prime minister and labour unions failed to break a political stalemate over a deeply unpopular pension bill making people work for longer.

Protests against the reform - which lifts the retirement age by two years to 64 - have drawn crowds of hundreds of thousands in rallies organised by unions since January, and at times turned violent.

Labour groups vowed to dig in their heels after talks with Prime Minister Elisabeth Borne on Wednesday - which lasted just an hour - failed to calm the situation.

They said the only way out of the crisis was for the legislation to be pulled, an option which Borne flatly rejected.

"After this meeting, I am calling for a maximum of workers, men and women, to join the marches across France tomorrow," Laurent Berger, head of the country's biggest union CFDT, said on Wednesday.

Thursday's marches - the 11th day of protests in the past three months - could provide an indication of whether the drawn-out rallies are losing steam or gaining momentum.

The previous day of demonstrations on March 28 drew smaller crowds, according to the Interior Ministry, with 740,000 people protesting across the country compared with a record 1.09 million seen on March 23.

Paris public transport operator RATP predicted traffic would be almost normal on Thursday. Trains are expected to be more heavily disrupted and the civil aviation authority asked airlines to cut flights by 20% in cities like Bordeaux and Marseille, but not at Paris airports like in the past.

Some 20% of primary school teachers are also expected to join the strike, local media quoted the Snuipp-FSU union as saying, down from 30% for March 28.

Strikes are still disrupting operations at oil refineries and nuclear plants, while garbage collectors have vowed to resume their protest from next week.

The latest wave of demonstrations represents the most serious challenge to the authority of President Emmanuel Macron, on a state visit to China, since the "Yellow Vest" revolt four years ago. Polls show a wide majority of French oppose the pension legislation and the government's decision to push it through parliament without a vote.