Hundreds of thousands of people demonstrated across France on Saturday, seeking to keep up pressure on the government over its pension reform plans, including a move to raise the retirement age to 64 from 62.

After three days of nationwide strikes since the start of the year, unions are hoping to match a mass turnout from January 19, when more than a million people marched in opposition to the plans.

The French spend the largest number of years in retirement among the Organization for Economic Cooperation and Development (OECD) countries - a benefit which, opinion polls show, a substantial majority of people are reluctant to give up.

President Emmanuel Macron says the reform is "vital" to ensuring the viability of the pension system.

The Interior Ministry said an estimated 963,000 people had attended demonstrations across the country. Paris police said some 93,000 people had marched in the capital, compared to 80,000 on January 19. The unions put the Paris figure at 500,000.

A car and some rubbish bins were set on fire in Paris' protests, and the police used teargas and stun grenades in their attempt to disperse some of the more radical elements in the protests.

There were protests in other French cities up and down the country, with images on television showing police using water cannons in the western city of Rennes.

In a joint statement ahead of Saturday's marches, all the main unions called for the government to withdraw the bill.

They warned that they would seek to bring France to a standstill from March 7 if their demands are not met. A strike is already scheduled for February 16.

"If the government continues to remain deaf, then the inter-union grouping will call for France to be shut down," they said ahead of Saturday's marches.

The protests first occurred over the weekend, when workers did not need to strike or take time off.

They follow the first week of debate on the pension legislation in parliament.

The opposition has suggested thousands of amendments to complicate the debate and ultimately try to force the government to pass the bill without a parliamentary vote and through decree, a move that could potentially sour the rest of Macron's mandate.

He was re-elected in April 2022 for five years.

Raising the retirement age by two years and extending the pay-in period would yield an additional 17.7 billion euros ($19.18 billion) in annual pension contributions, allowing the system to break even by 2027, according to the Labor Ministry estimates.

Unions say there are other ways to do this, such as taxing the super rich or asking employers or well-off pensioners to contribute more.

(With input from Reuters and AFP)