Six police officers were left injured after clashes between protesters throwing petrol bombs in Paris.
Riot police tried to control the rioting crowd by releasing tear gas, as tens of thousands of union activists took to the streets for May Day demonstrations.
One photographer reported seeing a riot officer engulfed in flames.
Interior Minister Matthias Fekl condemned the violence and said one officer was seriously burned on the hand while another had serious burns to the face.
Far-right presidential candidate Marine Le Pen tweeted: "This is the sort of mess... that I no longer want to see on our streets."
In a feisty speech, presidential candidate Emmanuel Macron told thousands of his supporters he would defend "free democracy" if voters choose him on Sunday.
The traditional union-led marches underlined the obvious absence of the united front shown in 2002 when Le Pen's father Jean-Marie shocked the country by reaching the run-off.
On this day 15 years ago, some 1.3 million people, including 400,000 in Paris, took to the streets of France in union-led demonstrations to protest against the founder of the National Front (FN).
That show of force, coupled with a political closing of ranks, helped the centre-right's Jacques Chirac inflict a crushing defeat on Le Pen senior.
This time, with left-wing candidates eliminated in the first round, the left is deeply divided over the choice between 48-year-old Le Pen and Macron, a 39-year-old former investment banker.
Macron is currently favourite to become France's youngest ever president, leading Le Pen by 19 points in the polls, but she has shown she is an effective campaigner.
Speaking at a convention centre in northern Paris, Macron, however, said Le Pen "had fed off worry and hate for years".
He said he was aware that "many people will vote for me to avoid having the National Front".
"I say to them that I am completely aware that on May 7, I will be doing more than defending a political programme, I will be leading the fight for the republic and for a free democracy," he said to roars of approval from supporters.