Trains cancelled, schools closed: France scrambled to make contingency plans on Tuesday for a huge strike against pension overhauls that poses one of the biggest challenges yet to President Emmanuel Macron's sweeping reform drive.

Paris metro and rail workers will walk off the job starting Thursday, banking on widespread support for their protest against plans they say will require millions to retire later or face reduced pensions.

It is the latest expression of discontent by broad swathes of French society, from police and firefighters to teachers and hospital employees -- as well as a yearlong "yellow vest" movement demanding improved living standards.

"The social discontent now is stronger than it was in 1995," according to Bernard Thibault, a former CGT leader during the protests 24 years ago.

The Paris metro is bracing for gridlock as 11 of the city's 16 lines shut down completely Thursday, with only the two fully automated lines running as normal.

Rail operator SNCF said 90 percent of high-speed TGV trains as well as regional trains across the country would be cancelled, while international services like the Eurostar and Thalys would be "severely disrupted."

Air France said it had cancelled a third of its domestic flights for Thursday as air traffic controllers walk off the job, along with 15 percent of mid-range flights to cities outside the country.

Photo: AFP / STEPHANE DE SAKUTIN

So far, Macron has been able to ride out opposition to previous signature reforms, including less-restrictive labour laws and tax cuts to encourage hiring and overhauls of the education system.

He even pushed through changes to the state-run rail operator SNCF, long considered an untouchable union bastion, ending job-for-life guarantees and other benefits for new employees.

Those successes have only fired up union leaders, who have vowed to maintain their strike until Macron abandons his campaign promise for a "universal" retirement system.

It would do away with 42 "special regimes" for sectors ranging from rail and energy workers to lawyers and Paris Opera employees, which often grant workers higher pensions or early retirement.

Photo: AFP / Lionel BONAVENTURE

But unions say the changes would effectively require millions of private-sector workers to work beyond the legal retirement age of 62 if they want to receive the full pension they have been promised.

"The flagrant provocation is trying to say this reform would affect only the special schemes," Philippe Martinez of the hard-line CGT union said last week.

"It's a shameful lie," he continued. "Everyone understands that it's a reform that will affect the public as well as the private sector."

Prime Minister Edouard Philippe has acknowledged French workers will gradually work longer but no concrete details of the overhaul are to be announced before mid-December.

And while 76 percent of respondents in an Ifop poll published on Sunday back the reform, 64 percent do not trust the government to pull it off.

Police unions, rubbish collectors, air-traffic controllers and Air France workers have said they will start striking on Thursday, and the biggest union for primary school teachers expects four out of 10 schools will remain shut.

Rallies and work stoppages are planned in dozens of cities, capped by two demonstrations planned for Paris that will converge on the Place de la Nation.

Officials have already ordered Paris businesses along the routes to close on Thursday, underscoring retailers' fears that a long strike will take a heavy toll on their businesses.

"It's a period of heavy spending," Francois Asselin, president of the CPME association of small and midsize businesses, told France Info radio on Tuesday.

"We estimate that a day of general strikes takes away some 400 million euros ($440 million) of revenues for the country, including nearly 200 million euros for the Paris region," he said.

And if the strike persists, he warned, "it can frankly be quite harmful, especially with Christmas coming up".