Paris commuters crammed into the city's sole two fully working Metro lines Thursday as labour strikes over pension reforms crippled France's train services for an eighth day and a major union warned the work stoppages could continue without pause until Christmas.

Tensions flared in several parts of the country. Police fired tear gas Thursday morning at protesters in the western city of Nantes, and protesters set vehicles on fire in the Mediterranean port city of Marseille.

Unions have flatly rejected fresh proposals by the government of President Emmanuel Macron to stagger the roll-out of the controversial plan that would require France's youngest workers to stay on the job two years longer, until the age of 64, to be eligible for a full pension.

"No Christmas break unless the government comes to its senses," Laurent Brun of the General Work Confederation union warned Thursday.

Workers across many sectors are participating in the nationwide general strike, including teachers, doctors, nurses, railway personnel, actors and garbage collectors.

The National Syndicate of High School Teachers said it was "stepping up" its actions against reforms it branded "unacceptable" with fresh protests across major French cities Thursday.

The unions' quick dismissal of the proposal the government made Wednesday signals that labour leaders were serious when they billed the strikes as "unlimited."

The strikes threaten to mirror similar ones in 1995 that lasted over three weeks, ended in the ouster of Prime Minister Alain Juppe and with proposed pension reforms being scrapped. The 1995 strikes were themselves compared in scale to the seismic events of May 1968, when France reached the brink of revolution.

Though Parisians have been adversely affected since they rely the most on public transportation, other parts of France have experienced less aggravation and the striking workers have enjoyed widespread public support, according to polls.

The mood among the public remains strained but understanding.

"I totally understand why people are on strike... Obviously, everyone is affected by the pension reform," London-Paris commuter Nicolas Lipitei said Thursday.

"(But) I think there should be better provisions to accommodate getting to work. There's a huge impact on the economy, a huge impact to people," he added.

Many French people and the unions leading the strikes fear the new system will force people to work longer for smaller pensions.

The government said the changes would ensure the pension system is "fair and sustainable" in the face of a growing population with a record number of people over age 90.

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Nicolas Garriga contributed