Paris - (AFP) - The French prime minister on Sunday ruled out a plan to raise the retirement age, as unions prepared for another day of protests and strikes on Tuesday against the controversial measure.

Increasing the minimum retirement age from the current 62 years to 64 years is part of a major reform package proposed by President Emmanuel Macron to ensure the financial balance of France's social insurance system.

After union protests against the programme, more than a million people took to the streets across France on January 19, and the government indicated there was scope to amend some measures.

This includes special conditions for those who started working at an early age, and others for mothers who put their careers on hold to take care of their children and for those who invested more in education.

But Prime Minister Elizabeth Born stressed Sunday that the minimum retirement age (64 years) is not subject to discussion. "This is not negotiable," Borne told France Info radio.

Unions welcome the government's willingness to negotiate parts of the plan, but stress the need to adjust the proposed retirement age.

In a rare agreement between them, the eight largest unions in France described the reform as "unfair" and said they hoped for "greater mobilization" on Tuesday, beyond the protests organized on January 19.

At the time, the government indicated the participation of 1.1 million people in the movement, while the unions spoke of the participation of more than two million.

"It looks like there will be more participants," said Celine Verzelletti, an official of the left-wing CGT federation.

The head of the moderate CFDT union, Laurent Bergé, said, "People strongly reject the project, and this opinion is spreading," referring to the results of opinion polls.

He warned that it would be a "mistake" for the government to ignore the mobilization.

Communist Party leader Fabian Roussel described Bourne's statement as "provocative", considering it "closed" and its government "inflexible".

And the leader of the far-right "National Rally" party, Marine Le Pen, reiterated her opposition to the government's "unfair and cruel" plans.

Both unions and the government see Tuesday's protests as a major test.

About 200 protests will be held across the country, with a large rally in Paris ending outside the National Assembly, where parliamentary committees are due to begin examining the bill on Monday.

The left-wing opposition submitted more than 7,000 proposals to amend the draft in an attempt to slow down its debate in Parliament.

Macron and his allies lack an outright majority in parliament and will need votes from conservatives to pass the pension plan.

But the government has the option of imposing the bill without a vote under special constitutional powers, which means it risks causing a vote of no confidence and possibly triggering new parliamentary elections.

On Sunday evening, Bourne held a meeting with a number of its ministers and senior government officials to discuss the next steps.

In addition to the protest marches, the unions called for a large-scale strike for Tuesday, which is expected to severely affect railway services and the rest of public transport.

Schools and administrations are also expected to stop, and some local authorities have announced the closure of public spaces such as sports stadiums.

On Sunday, Transport Minister Clement Bonn warned those using public transportation that Tuesday will be "difficult, even very difficult," calling on them to postpone their travels and work from home if possible.

Some unions called for more strikes in February, in sectors including commercial ports, oil refineries and gas stations.

Observers believe that the unions are counting heavily on the success of the mobilization, and that any slowdown in support on Tuesday may undermine the momentum of the protest.

In this regard, political science professor Dominic Andolfato said that the unions have "raised the bar of expectations" and therefore "cannot afford to stumble".