Traditional May 1 union marches turned violent in France on Monday and presidential election frontrunner Emmanuel Macron attacked far-right rival Marine Le Pen, highlighting the divisions six days before the runoff.

Six police officers were hurt in clashes in Paris between masked youths throwing molotov cocktails and riot police who responded with tear gas, as tens of thousands of union activists took to the streets for May Day demonstrations.

One riot police officer was engulfed in flames, an AFP photographer saw.

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, without saying which was the officer in the photograph.

Reacting to the scenes, far-right presidential candidate Le Pen tweeted: "This is the sort of mess... that I no longer want to see on our streets."

In a feisty speech, Macron told thousands of his supporters he would defend "free democracy" if voters choose him on Sunday after Le Pen had urged voters to reject "the world of finance, of arrogance, of money as king" she said her opponent embodied.

The traditional union-led marches underlined the conspicuous 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 Le Pen's 48-year-old daughter and Macron, a 39-year-old former investment banker.

In Paris's Place de la Republique, 28-year-old teacher Camille Delaye held a placard that read: "Abstention is a political act" -- meaning he is so disgusted by both candidates, he will not vote on Sunday.

But Nastassja Naguszewski, 28, who works in local government, said: "You have to put Le Pen as low as possible. One of them is a candidate who will uphold republican values and one is not."

'Le Pen is anti-workers'

Two unions, the CFDT and Unsa, have called for their members to back Macron.

But while three other more left-wing unions including the biggest, the CGT, have called for demonstrations against Le Pen's vision of French identity, they have stopped short of backing Macron whose economically liberal outlook worries many members.

Some militants have formed a movement they have called "Social Front" to block both candidates.

CGT leader Philippe Martinez said he "deeply disagreed" with that approach, arguing that Le Pen and Macron "are not the same thing".

"The National Front is a racist, xenophobic party that is anti-women and anti-workers because it is also an economically liberal party," he said.

Le Pen hit back that the unions "are not defending workers' interests, they are looking after their own interests".

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".

'More than politics'

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.

Le Pen took her campaign Monday to the working-class Paris suburb of Villepinte where the first speaker was Nicolas Dupont-Aignan, a eurosceptic from outside the FN who Le Pen has said will be her prime minister if she wins on Sunday.

She is hoping to capture some of the 1.7 million votes he won in the first round, although his support for Le Pen has divided his own party.

"It's a choice between France and finance," Dupont-Aignan said in a jibe at Macron.

In her remarks, Le Pen repeated nearly verbatim several passages from a speech made two weeks earlier by Francois Fillon, the former frontrunner who was eliminated in the first round.

Asked by AFP about the apparent plagiarism, FN deputy leader Florian Philippot said it was "a nod to a short passage in a speech about France" on the part of "a candidate that shows she is not sectarian".