Paris police on Thursday used tear gas to disperse rioters who set fire to a vehicle and smashed windows at a protest marking one of the biggest strikes in France in years, AFP correspondents said.
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.
According to Paris police, 71 people have been detained so far in the French capital. The protest was still continuing as darkness began to fall with demonstrators still thronging the square.
At least 450,000 people took to the streets across France -- not including Paris -- according to an AFP count based on police and official figures. Most protests were peaceful.
The CGT union said 250,000 people protested in the French capital.
The so-called "yellow vest" protesters whose weekly demonstrations have rocked the rule of President Emmanuel Macron over the last year have vowed to join in the union-led strike protests.
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However, analysts have warned that this risks seeing the movement spiral out of the control of the unions, should the protests turn violent.
There were tensions in the northwestern city of Nantes where a total of 20,000 people turned out and police used tear gas against the most radical protesters known as the "black blocs".
Police and demonstrators also clashed in the southeastern city of Lyon, where security forces also used tear gas.
The demonstrations spread as far as France's overseas departments, with some 3,000 people protesting in Saint-Denis, the main city of the French Indian Ocean territory of La Reunion.