Three riot police officers have been injured in clashes between masked protesters on the fringes of May Day marches in Paris.
Scores of hooded youth threw Molotov cocktails at security forces who fired back with tear gas during the otherwise peaceful march on Monday. Police responded by firing tear gas, making four arrests of individuals for "carrying prohibited weapons".
"Three officers have been injured, including one seriously due to burns from Molotov cocktails," said a police source.
"These people are here to smash things up, not to protest," said the UNSA police union.
Video showed riot police surrounding around 150 protesters disrupting the march after cordoning off most of them from the rest of the crowd near the Place de la Bastille. However, some continued to lob firebombs that exploded into flames in the street.
Loud explosions, apparently from large firecrackers, rang out across central Paris. Several shops and a bus shelter were vandalised.
More than 9,000 armed police and soldiers were drafted in to try to keep the peace as union members and protesters took part in annual May Day marches to celebrate workers' rights, which this year included calls to block far-right presidential candidate Marine Le Pen from winning the presidency during a runoff election on Sunday.
Both Ms Le Pen and her centrist opponent Emmanuel Macron are holding rallies today.
However, splits emerged within the major unions over what stance to adopt regarding the presidential candidates.
Some factions are going against their leadership to call for members to vote "neither (for Ms Le Pen) nor (for Mr Macron)" - seen by many leftists as an enemy of the worker. A banner of one dissenting faction of the CGT union reportedly read: "Neither plague nor cholera."
Philippe Martinez, CGT leader, said he was in "deep disagreement" with this stance, saying the official union line is "not a single vote for Marine Le Pen."
"Our slogan is clear. The FN must be beaten for social progress," said Mr Martinez. "The FN is a racist, xenophobic, anti-women and liberal party," he said.
Seizing on the violence by a small minority in otherwise peaceful Mayday marches, Ms Le Pen tweeted: "I pay tribute to the two (sic) injured CRS (riot police). It's is this mess and this laxism that I don't want to see anymore in our streets."
Je rends hommage aux deux CRS blessés à Paris. C'est cette chienlit et ce laxisme que je ne veux plus voir dans nos rues. MLP— Marine Le Pen (@MLP_officiel) May 1, 2017
Jean-Luc Mélenchon, the leftist leader of La France Insoumise (Unbowed France) who came fourth in the first presidential round, took part in one of the Paris marches.
Ms Le Pen spent last week trying to woo Mélenchon voters, whose statist, protectionist policies she insists have many crossovers with her own. Mr Macron's pro-business programme was "diametrically opposed" to leftist ideals, she insisted.
However, after remaining silent for a week, the hard-Left firebrand finally came out on Sunday to urge his supporters not to vote for Ms Le Pen.
"I say to anyone who is listening: do not make the terrible error of voting for the Front National because you would push the country towards a general conflagration and the ending to which no-one can predict," he said on the TF1 television channel.