France's prime minister on Sunday ruled out backtracking on a plan to raise the retirement age as unions prepared for another day of mass protests and strikes against the contested reform.

An increase in the minimum retirement age to 64 from the current 62 is part of a flagship reform package pushed by President Emmanuel Macron to ensure the future financing of France's pensions system.

After union protests against the change brought out more than a million people into the streets across France on January 19, the government signalled there was wiggle room on some measures.

They included special deals for people who started working very young, and provisions for mothers who interrupted their careers to look after their children and for people who invested in further education.

But the headline age limit of 64 is not up for discussion, Prime Minister Elisabeth Borne said Sunday.

"This is now non-negotiable," she told the FranceInfo broadcaster.

While unions have welcomed the government's readiness for negotiation on parts of the plan, they say the proposed 64-year rule has to go.

Calling the reform "unfair", France's eight major unions, in a rare show of unity, said they hoped to "mobilise even more massively" on Tuesday -- their next scheduled protest day -- than at the January 19 rallies.

On that occasion, the government put the turnout at 1.1 million; the unions said more than two million.

"It's looking like there will be even more people," said Celine Verzeletti, member of the hard-left union CGT's confederation leadership.

Pointing to opinion polls, Laurent Berger, head of the moderate CFDT union, said "the people disagree strongly with the project, and that view is gaining ground".

It would be "a mistake" for the government to ignore the mobilisation, he warned.

Communist party leader Fabien Roussel called Borne's remark "a provocation", saying the prime minister was "blinkered" and her government "inflexible".

Marine Le Pen, leader of the far-right National Rally, repeated her opposition to the government's "unjust and brutal" plans.

Unions and the government both see Tuesday's protests as a major test.

Some 200 protests are being organised countrywide, with a big march planned for Paris culminating in a demonstration outside the National Assembly where parliamentary committees will start examining the draft law Monday.

The left-wing opposition has submitted more than 7,000 amendments to the draft in a bid to slow its path through parliament.

Macron's allies, short of an absolute majority in parliament, will need votes from conservatives to get their pensions plan approved.

The government has the option of forcing the bill through without a vote under special constitutional powers, but that risks triggering a vote of no confidence, and possibly new parliamentary elections.

On Sunday evening, Borne held a meeting with several of her ministers and senior government officials to discuss how to proceed.

- Strike calls -

In addition to protest marches, unions have called for widespread strike action for Tuesday, with railway services and public transport expected to be heavily affected.

Stoppages are also expected in schools and administrations, with some local authorities having already announced closures of public spaces such as sports stadiums.

For people using public transport, Tuesday will be "difficult, or even very difficult", Transport Minister Clement Beaune said Sunday, calling on commuters to postpone trips and work from home when possible.

Some unions have called for further strike action in February, including at commercial ports, refineries and power stations.

Observers say the unions are playing for high stakes, and any slackening of support Tuesday could be fatal for their momentum.

"They have placed the bar high," said Dominique Andolfatto, a professor of political science. "They can't afford any missteps."