The pensions reform is due to be passed by the Senate Wednesday and another day of strikes and demonstrations is planned for Thursday.
Protests last week in Lyon and some other cities saw groups of youths smash shop fronts, burn cars and fight with police.
Now Bernard Thibault, the leader of the powerful CGT union federation, has backed accusations by hard-left politician Jean-Luc Mélenchon that police in plain-clothes were among the casseurs – as the French call young rioters - and that they also infiltrated strike pickets and occupations.
“The presence of police on duty camouflaged with union badges in Lyon and Paris is beyond doubt,’ Thibault told the Libération daily, which carries videos purporting to prove the presence of provacateurs.
“Manipulators infiltrate and incite crime on strike pickets, demonstrations and occupations of roundabouts, incite violence at the end of demonstrations so as to have sensationalised images for the TV news at 8pm and create a climate of tension,” Thibault said.
On Sunday Mélenchon, who heads the Parti de Gauche which split from the Socialist Party in 2009, claimed that infiltrators “throw stones, break windows and then take out police armbands” in a television news interview.
Interior Minister Brice Hortefeux leapt to the police force’s defence, branding Mélenchon’s remarks “unacceptable” but declined to sue him on the grounds that it would give the leftwing leader more publicity.
Police unions, who had called for legal action, called the accusations “scandalous” and “demagogic”, claiming that the left is trying to distract attention from the fact that some strikers are returning to work.
Hortefeux said that 2,254 people had been arrested on protests on 12 October and that official figures of 72 police officers injured are an underestimate.