More than 800,000 people have marched in cities across France as railway workers, teachers and hospital staff held one of the biggest public sector strikes in decades against Emmanuel Macron’s plans to overhaul the pension system.

A nationwide transport strike brought much of France to a standstill and was expected to continue for the next few days as unions dug in, saying the president’s pension changes would force millions of people to work longer or receive lower payments.

Trains, metros and bus services were severely hit, some flights were cancelled and many schools were closed in the biggest challenge to Macron’s reform agenda since the gilets jaunes (yellow vest) anti-government protests erupted last year.

In Paris, police briefly fired teargas during skirmishes with black-clad protesters on the edges of the trade union-led march in the early afternoon. Some protesters set fire to a storage trailer, smashed windows and a bus shelter, and overturned cars. Firefighters put out small fires lit in bins.

More than 6,000 riot police lined the route of the demonstration from Paris Gare du Nord to the east of the city, while groups of officers stopped people walking towards the demonstration and searched bags. By late afternoon there had been more than 70 arrests and 9,000 searches.

Riot police in Nantes, western France, fired teargas at masked protesters, who hurled projectiles at them.

The Paris march included hospital staff, electricity workers, firefighters, teachers and school pupils as well as gilets jaunes protesters who had taken part in blockades on roundabouts earlier this year. Banners read: “Macron out.”

Isabelle Jarrivet, 52, who had worked as an administrator in a town hall north of Paris for 20 years, said: “It’s a question of life or death for the French social system, which Macron is dismantling. We’re being taken back to a time before 1945, where we risk losing the social safety net. Private pension funds are waiting in the wings to benefit.”

She added: “The gilets jaunes protests got people thinking and talking more about politics and people determined not to let things pass. You can feel a defiant mood in the air.”

The standoff is a crucial test for the centrist president, whose planned overhaul of the pensions system was a key election promise. Macron’s office said he was following events closely “with calm and determination”.

The government argues that unifying the pensions system – and getting rid of the 42 “special” regimes for sectors ranging from rail and energy workers to lawyers and Paris Opera staff – is crucial to keep the system financially viable as the population ages. But unions say introducing a universal system for all will mean millions of workers in both the public and private sectors must work beyond the legal retirement age of 62 or face a severe drop in the value of their pensions.

The row cuts to the heart of Macron’s presidential project and his pledge to deliver the biggest transformation of the French social model and welfare system since the postwar era. Since his election in 2017, Macron has leaned towards a Nordic style of “flexi-security” in which the labour market is loosened and the focus is on changing from a rigid work code to a society of individuals moving between jobs.

While many voters agree the pensions system should be changed, they are not sure the pro-business president can be trusted to do it. A recent Ifop poll found 76% of people back pensions reform, but 64% do not trust the government to pull it off.

Public transport unions said they would extend their strike until at least Monday, after 90% of TGV and regional trains were cancelled and nearly all lines on the Paris metro were affected.

Sandrine Berger, an engineering lecturer at a Paris university and a representative for the leftwing CGT union, said: “This is about protecting public services, which are being chipped away and turned towards an American model of privatisation.”

She criticised the government for suggesting public sector workers were privileged. With two PhDs, a 26-year career and a senior job running a department of 70 people, Berger said she earned €2,200 (£1,860) a month before tax, including bonuses.

Grégory Chaillou, a fire officer, said: “We’ll see this through to the end. The public understands the need to support public services and workers.”

As commuters in Paris turned to using bikes and scooters, the environmental group Extinction Rebellion claimed responsibility for the sabotage of 3,600 electric scooters in Paris and other French cities, saying the green image of the fashionable gadgets hid an “ecologically catastrophic” reality.

Extinction Rebellion said it had sabotaged the scooters, including more than 2,000 in Paris as well as in Bordeaux and Lyon, by obscuring the QR codes that riders use to unlock them with their smartphones.

“Contrary to their reputation as a ‘soft’ or ‘green’ way of getting around, the electric scooters are ecologically catastrophic,” the group said in a statement on its French Facebook page.