A nationwide strike brought much of France to a standstill in protest against the government's planned overhaul of the pension system.
The new system is one of President Emmanuel Macron most ambitious reforms.
The CGT union said 1.5 million people hit the streets nationwide in protest while the Interior Ministry put the number at 806,000.
The huge demonstrations across the country added to walkouts that paralysed transport services and closed schools, with international high-speed rail disrupted and flights cancelled.
In the capital as well as in the southeastern city of Lyon and western city of Nantes police fired teargas to disperse small groups of rioters but protests were mainly peaceful.
Mr Macron's government has yet to set out its reform proposals in full, but is pushing for a single plan to replace 42 special pension schemes.
Striking transport workers, air traffic controllers, teachers, fire fighters, lawyers and other groups all fear they will have to work longer or receive reduced pensions under the plans.
"We haven't seen such a turnout in a very long time," said Yves Veyrier, head of the Force Ouvriere union at the beginning of the Paris rally.
"We now expect the government to take the measure of this mobilisation, and understand that its universal system is a bad idea," he said.
Prime Minister Edouard Philippe praised trade unions for keeping the protests on a tight rein, minimising the violence.
The disturbances looked set to continue for days, sorely testing Mr Macron's determination to implement a key campaign promise.
The national railway company SNCF cancelled 90% of high-speed trains for tomorrow and warned of "very severe disruptions" for the Eurostar and Thalys trains serving London and Brussels.
Paris transport workers, meanwhile, voted to extend their industrial action, which shut down 11 out of 16 metro lines, until at least Monday.
Air France called off 30% of its domestic flights and 10% of its short-haul international routes.
Customer Update - French National Strike, Friday 6th December 2019: pic.twitter.com/NuydvgTC5V— Ryanair (@Ryanair) December 5, 2019
EI524 (Dublin-Paris) and EI525 (Paris-Dublin) on Friday 06 December are cancelled due to the national strike in France. We're working to re-accommodate affected guests. Those opting to cancel their trip will be refunded. Please check flight status before journeying to the airport— Aer Lingus (@AerLingus) December 5, 2019
The demonstration in Paris got off to a tense start with black-clad protesters setting fire to a storage trailer and smashing storefront windows but the main march passed off peacefully.
Many people simply took the day off to avoid the travel chaos, with several stores and restaurants shuttered because employees could not get to work.
Bike paths were packed as commuters turned to bicycles and electric scooters, while several ride-hailing companies were offering special strike promotions.
National newspapers were unable to publish their print editions, and the CGT union said workers had blocked seven of the country's eight oil refineries, raising the prospect of fuel shortages if the strike continues.
The strike has revived memories of the three-week-long strikes over pension reforms that crippled France in 1995, forcing the government of the day to reverse course.
The minimum pension age in France is 62, one of the lowest among developed countries.
But railway workers, opera employees and a host of other workers have more advantageous schemes, with train drivers for example generally retiring in their early 50s.
A pre-school teacher demonstrating in the eastern city of Belfort, Anne Audier-L'Epingle, told AFP she was striking because "I don't know what I will be able to offer two-year-old children when I'm 65."
Mr Macron has faced a wave of opposition this year, from the "yellow vest" movement demanding improved living standards to more recent protests from police, firefighters, teachers, hospital workers and lawyers.
So far the former investment banker has largely succeeded in pushing through a series of controversial reforms, including loosening labour laws and tightening access to unemployment benefits.
But this is the first time the various disgruntled groups have come together in protest.
"I'm not just here about pensions - the health service is our business, public services and also the general drift of the government to the right in general," said Pierre Malige, a 42-year-old professor of mechanics at the University of Nantes, at a protest in the city.
Travellers are facing the prospect of days of chaos, with the SNCF already halting all train ticket sales until Monday.
The Eiffel Tower and the Orsay museum were shut because of staff shortages, while the Louvre, the Pompidou Centre and other museums warned that some wings and exhibits were closed.
In an attempt to harness the momentum of the strike, the yellow vests, whose weekly protests have dwindled in size recently, blocked roads in several cities including the port of Le Havre on the Atlantic coast.