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.
The latest opinion poll showed Macron leading Le Pen by 61 percent to 39 ahead of Sunday's election, which offers France a choice between his vision of closer integration with a modernized 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 program 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 himself a year ago.
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.
Campaigning in Villepinte, a suburb north of the capital, Marine Le Pen told a rally: "Emmanuel Macron is just Francois Hollande who wants to stay and who is hanging on to power like a barnacle."
Since taking over the party, she has worked hard to cleanse it 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 French mediaeval heroine Joan of Arc, just a few hundred yards (meters) 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.
The bitterly contested election has polarized France, exposing 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 who is from neither of the main political groupings: the candidates of the Socialists and conservative party The Republicans 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. That has fed speculation that a high abstention rate could favor Le Pen, whose supporters typically tell pollsters they are staunchly committed to their candidate.
In recent days, Le Pen has sought to play down the importance of an exit from the euro - the part of her campaign platform that is the least popular with voters.
In her speech lasting nearly an hour on Monday, she made no reference to the single currency, devoting most of it to slamming her centrist rival and billing him as the candidate of the establishment.
"I want France to get its independence back by negotiating with Brussels the return of our sovereignty," she said.
Referring to her plan to hold a referendum on whether France should remain in the EU, she said: "The French people will decide."
Macron was due to speak at a rally in Paris later on Monday.