Meanwhile, the traditional May 1 union marches across France celebrating workers' rights will be politically charged this year.
One photographer reported seeing a riot officer engulfed in flames.
Marine Le Pen's campaign team is attempting to shake off ridicule over her alleged plagiarism of a speech by Francois Fillon, just days before the French presidential election.
Macron insisted that despite Marine Le Pen's efforts to distance herself from her father's anti-Semitism, "the roots are there, and they are very much alive".
The ministry said Macron used "unacceptable comparisons and mental shortcuts that lead to errors in public opinion" and denied that Poland is an ally of Le Pen.
Masked youths threw molotov cocktails.
The moderate CFDT union, or French Democratic Confederation of Labour, held a small rally against Ms Le Pen in Paris, while protesters at the larger march nearby carried signs that read: "Let's block the National Front".
Mainstream parties on the left and right failed to form a bloc against her as they did in 2002 when Jean-Marie Le Pen was trounced by Jacques Chirac.
Numerous protesters who took to the streets Monday were reportedly there to show their distaste for both far-right candidate Marine Le Pen and centrist Emmanuel Macron. The latest polls have shown an increase in the number of voters who say they plan to abstain in the second round.More news: Sources Indicate Liverpool Are Closing In On £26m Deal For Gremio Forward
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The accusations came as supporters of her rival Emmanuel Macron warned that a high abstention rate could hurt his chances of victory in the second round vote on Sunday.
"Emmanuel Macron is just Francois Hollande who wants to stay and who is hanging on to power like a barnacle", she told a rally in Villepinte, a suburb north of the capital.
As recently as early April the unceasingly controversial woman who could become France's first female president told her supporters she would "uncompromisingly fight Islamist fundamentalism which seeks to impose its oppressive rules in our country".
Less than a week before Sunday's runoff, far-right leader Marine Le Pen and centrist Emmanuel Macron are holding separate rallies Monday.
He said he was aware that "many people will vote for me to avoid having the National Front".
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.
He said only his En Marche movement could "remake" French politics.
A couple and their two children marching against Le Pen in central Paris said: "The situation is serious enough that we're out on the streets; it's like people have forgotten it's the Front National", said their father David.
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.