French police fired tear gas and charged hooded protesters on Thursday at a demonstration in Paris against President Emmanuel Macron's pension reform plan.
The tensions broke out close to the place de la Republique square in the east of Paris, where thousands had massed to protest planned pension reforms.
A construction trailer was overturned and set on fire, sending a huge plume of smoke into the sky.
Authorities said around 285,000 people had taken part in rallies across the country by midday, a figure that did not include the several thousand at a Paris march that began at 2:00 pm.
Paris deployed 6,000 police for the march.
Railway workers, teachers and emergency room medics launched on Thursday one of the biggest public sector strikes in France for decades, determined to force Macron to abandon plans to overhaul the country's generous pension system.
Public transport unions said they would extend until at least Monday their open-ended strike.
Union leaders are demanding that Macron scrap his plan for a "universal" system that 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.
Critics say the changes would effectively require millions of workers in both the public and private sectors to work beyond the legal retirement age of 62 if they want to receive the full pension they have been promised.
French rail operator SNCF said Thursday that it had cancelled 90 percent of all high-speed TGV trains and 70 percent of regional trains for Friday amid the strike.
SNCF said that services would "still be very disrupted" on the second day of the biggest transport strike in the country in years, with the Eurostar service to Britain and the Thalys service to northern Europe set to be "very heavily disrupted".
Also French Civil Aviation Authority instructed airlines to reduce their flight scheduled by 20% on Friday, Dec 6, to and from Charles De Gaulle, Paris-Orly and regional airports.
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.
And "yellow vest" protesters, who have staged weekly demonstrations since last year demanding improved living standards, blocked roads in several cities including Le Havre on the Atlantic coast, where the heavily used port was also shut according to the CGT.
*This story was compiled by Ahram Online