Tear gas floats around masked protesters during clashes with French riot police. (AFP)
Paris riot police fired teargas as they squared off against hardline demonstrators among tens of thousands of May Day protesters, who flooded the city Wednesday in a test for France's zero-tolerance policy on street violence.
Tensions were palpable as a heady mix of labour unionists, "yellow vest" demonstrators and anti-capitalists gathered in the French capital, putting security forces on high alert.
Ahead of the main march, the city was on lockdown with more than 7,400 police and gendarmes deployed with orders from President Emmanuel Macron to take an "extremely firm stance" if faced with violence.
Clashes briefly erupted on Montparnasse Boulevard, where hundreds of anti-capitalist "black bloc" activists pushed to the front of the gathering crowd, hurling bottles and other projectiles at police, who fired tear gas and stingball grenades, an AFP correspondent said.
Used at ground level, the grenades release scores of rubber pellets that cause an intense stinging to the legs.
Authorities had warned this year's marches would likely spell trouble, coming barely a week after leaders of the yellow vest anti-government movement angrily dismissed a package of tax cuts by President Emmanuel Macron.
And with some agitators vowing on social media to turn Paris into "the capital of rioting", the government moved to deploy security on an "exceptional scale" throughout the capital.
Last year, officials were caught off guard by some 1,200 troublemakers who ran amok in Paris, vandalising businesses and clashing with police.
By early afternoon, thousands had flocked to the Montparnasse area, many wearing the hi-visibility jackets that gave the name to the yellow vest protesters.
Since November, the city has struggled to cope with the weekly yellow vest protests, which have often descended into chaos with a violent minority smashing up and torching shops, restaurants and newspaper stands.
Across the city on Wednesday, streets were barricaded and shops had boarded up their windows, with police ordering the closure of all businesses along the route of the main march.
"We are not afraid of the union marches but of the black blocs," local restaurant owner Serge Tafanel told AFP.
"Capital of rioting"
Interior Minister Christophe Castaner said several groups on social media had urged protesters to transform Paris into "the capital of rioting", with police gearing up for the arrival of up to 2,000 activists bristling for a fight.
Many are anti-capitalist youths, often known as black blocs, who dress in black and wear face masks.
Nearly 200 motorcycle units have been deployed across the capital to respond quickly to flare-ups of violence, and drones are being used to track protesters' movements.
Castaner said police would carry out pre-emptive searches of anyone planning to march, a new tactic allowed under a security law passed recently in response to the yellow vest violence.
From the early hours of Wednesday, several dozen police officers could be seen at the city's main train stations, carrying out bag random searches, AFP journalists said.
By early afternoon, police said 165 people had been detained for questioning.
Sidelined, unions seek visibility
Last Thursday, in a major policy speech aimed at calming the yellow vest anger, Macron promised a string of reforms including tax cuts worth five billion euros ($5.5 billion).
The yellow vests rejected it as too little, too late, pledging to keep up the protests, which began last year over rising taxes on fuel and pensions but have since morphed into a wider movement.
Although the numbers have steadily fallen, the rallies have remained in the headlines, largely over disorder by a handful of violent protesters along the Champs-Elysees.
Following a particularly violent demo in March, the government adopted a "zero-tolerance" approach, passing an "anti-rioter" bill which included making it a criminal offence to wear a mask at a protest.
France's powerful labour unions are also hoping to use the traditional May Day march for workers' rights to raise their profile after finding themselves sidelined for months by the grass-roots yellow vest movement.
Like the yellow vests, the unions were also disappointed by Macron's speech.
"We must be careful not to lose the meaning of this day," warned Philippe Martinez, secretary general of the CGT, one of France's biggest unions.
"It is a day of mobilisation which deserves our full attention after Emmanuel Macron's announcement in which he said: 'I hear you and I'm not changing anything'."
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