One officer was engulfed in flames, an AFP photographer saw, and unconfirmed reports said he had suffered serious burns.
Varoufakis wrote that it was a "veritable mystery" why "certain leaders of the left" had not yet favored the centrist candidate Macron over the far-right Le Pen in the second round of the French presidential election scheduled on May 7.
Masked youths threw molotov cocktails.
As May Day marchers prowled and clashed on the streets of Paris.
Contacted by Le Monde the same day, the director of Le Pen's campaign, David Rachline, said that he was not aware of this plagiarism, or who the author of the speech was.
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.
He says Le Pen's priorities as president would be "to fight against press freedom, "against women's rights, the right to abortion" and "against same-sex couples' rights". One of them is a candidate who will uphold republican values and one is not".
She called for France to reclaim its "independence" from the European Union but made no mention of her proposal to drop the euro, the part of her campaign platform which is the least popular with voters, and which she has played down in recent days.
Ms Le Pen added to Mr Fillon's comment that this showed France is more than a military and industrial power, that France "must and can" become a big power again.
She told the rally: "Do not give a voice to Mr Macron, do not prolong this unworthy five-year term, do not prolong the sufferings of a people that can not longer cope. I think this little wink was appreciated, including by supporters of Mr Fillon".
Liberation said it would publish a 16-page anti National Front (FN) special edition on Tuesday. Four officers were injured, with one seriously burned in the face, Interior Minister Matthias Fekl said.
Le Pen hit back that the unions "are not defending workers' interests, they are looking after their own interests".
Macron was due to speak at a rally in Paris later on Monday.
He said only his En Marche movement could "remake" French politics.
At her rally Monday in Villepinte, Le Pen repeated nearly verbatim these passages from Fillon's speech.
Macron sought for a third successive day to paint National Front (FN) leader Le Pen as an extremist, while she portrayed him as a clone of unpopular outgoing Socialist President Francois Hollande, under whom he served as economy minister from 2014 to 2106.
The 88-year-old was kicked out of the FN by his daughter two years ago for repeatedly calling the Holocaust a "detail of history" - remarks which were preventing Marine Le Pen from putting distance between the party and its past links with anti-Semitism.
Ms Le Pen, 48, has been holding her rally in the working-class Paris suburb of Villepinte.
The son of a man killed on the sidelines of a 1995 far-right march has joined French presidential candidate Emmanuel Macron on a Paris bridge.