Macron sought for a third successive day to paint National Front (FN) candidate 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 2016.
Two opinion polls showed Macron winning what is widely seen as France's most important election in decades with between 59 and 61 percent of the vote.
On offer is a choice between his vision of closer integration with a modernised European Union and her calls to cut immigration and take the country out of the euro.
"I will fight up until the very last second not only against her programme but also her idea of what constitutes democracy and the French Republic," said Macron, an independent backed by a new party, En Marche! (Onwards!), which he set up a year ago.
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Violence mars France's pre-election May Day marches
He was speaking after paying tribute to a young Moroccan man who drowned in the River Seine in Paris 22 years ago after being pushed into the water by skinheads on the fringes of a May Day rally by the FN, then led by Le Pen's father Jean-Marie.
Later, at a rally, he described her as "the anti-France candidate" and said "We want Europe to be strong because we want France to be strong."
He also rejected calls from leftwing politicians to drop his business friendly reforms.
In Villepinte, a suburb north of the capital, Marine Le Pen told supporters: "Emmanuel Macron is just Francois Hollande who wants to stay and who is hanging on to power like a barnacle."
She called for France to reclaim its 'independence' from the EU 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.
Party image
Le Pen has worked hard to cleanse the FN of xenophobic and anti-Semitic associations and make it more appealing to a wider electorate. She said at the weekend she had no more contact with her father and was not responsible for his 'unacceptable comments'.
Le Pen senior gave his own traditional May Day speech at a statue of national heroine Joan of Arc, a short walk from where Macron commemorated the death of young Moroccan Brahim Bouarram.
"Emmanuel Macron is doing a tour of graveyards. It's a bad sign for him," he said before a speech to a few hundred supporters that drew chants of "France for the French!" and "Islam out of France!"
At rallies in Paris, Marseille and elsewhere, some trade unionists and activists sought to turn the May Day workers' holiday into a day of national solidarity against the National Front, mirroring protests in 2002 when Jean-Marie Le Pen reached the second round of the presidential election before losing heavily to conservative Jacques Chirac.
However, other groups, including the powerful CGT union, refused to explicitly back Macron and organised their demonstrations separately.
Groups of masked and hooded youths dressed in black were visible among protesters in the capital. Protesters hurled petrol bombs and police responded with tear gas, making five arrests. Six police officers were hurt, one suffering serious burns.
Divided France
The election has exposed some of the same sense of anger with globalisation 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."
