One French police officer was seriously burnt and two others injured in clashes at a May Day demonstration in Paris on Monday in which protesters threw Molotov cocktails and other missiles, the police said.
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Television pictures showed policemen trying to shake flames from their riot gear, and of tear gas enveloping the streets around Paris' Bastille monument.
According to the Guardian, the street battle with the police began shortly after the Paris march, attended by thousands of participants, set off from Place de la Republique. A group of some 150 masked protesters armed with Molotov cocktails, stones and sticks forced their way to the front of the march and started lobbing missiles at the police, which responded with tear gas.
This year's May Day came less than a week ahead of the final round of a presidential election where voters must choose between the far-right National Front's Marine Le Pen and centrist Emmanuel Macron.
Some trade unionists and left-wing activists sought to make the day one of national solidarity against the National Front, mirroring protests in 2002 when Le Pen's father, party founder Jean-Marie Le Pen, was a candidate.
Marine Le Pen tweeted her support for the injured policemen and said the incident was the type of unacceptable behavior that she no longer wanted to see on French streets.
The election has exposed some of the same sense of anger with globalization and political elites that brought Donald Trump to presidential power in the United States, and caused Britons to vote for a divorce from the EU.
The vote in the world's fifth largest economy, a key member of the NATO defense alliance, will be the first to elect a president from neither of the main political groupings. The Socialist and conservative party candidates were knocked out in the first round on April 23.
Between them Le Pen and Macron gathered only 45 percent of votes in that round, which eliminated nine other candidates.
The second round will take place in the middle of a weekend extended by a public holiday. A high abstention rate could favor Le Pen, whose supporters typically tell pollsters they are staunchly committed to their candidate.
She devoted much of her speech, lasting nearly an hour, to attacking Macron as the face of the establishment. He was due to speak at a Paris rally later in the day.
Referring to her plan to hold a referendum on whether France should remain in the EU, Le Pen said: "The French people will decide."