French far-right leader Marine Le Pen has refused to back either President Nicolas Sarkozy or Socialist Francois Hollande in Sunday's run-off.
She vowed to cast a blank ballot and told supporters of her National Front to vote with their conscience.
Although it was expected, analysts say the move deals a further blow to Mr Sarkozy's re-election chances.
He needs the majority of the 6.4m voters who voted National Front in the first round to back him in the second.
The latest opinion polls suggest Mr Hollande has a six to 10 point lead over President Sarkozy of the centre-right UMP party.
The two will go head-to-head in the sole televised election debate on Wednesday.
Ms Le Pen took over from her father Jean-Marie as head of the National Front (FN) just over a year ago.
In the first round on 22 April, she won 17.9% of the vote - the best-ever score by a FN leader in a presidential election.
Addressing the party's annual rally at Place de l'Opera in Paris, she said: "Hollande and Sarkozy - neither of them will save you."
She accused both candidates of surrendering to Europe and financial markets.
"Who between Francois Hollande and Nicolas Sarkozy will impose the austerity plan in the most servile way? Who will submit the best to the instructions of the IMF, the ECB or the European Commission?" she asked.
"On Sunday I will cast a blank protest vote. I have made my choice. Each of you will make yours."
The most recent opinion polls suggest 44% of National Front voters will back the president, 38% will abstain and 18% will support Mr Hollande.
Analysts have calculated that Mr Sarkozy - whose five-year presidency has been overshadowed by the financial and the euro crises - might need as many as 80% of Ms Le Pen's first-round voters to win.
Later on Tuesday, Mr Sarkozy held a large rally in Paris' Trocadero Square to highlight "real work".
BBC Europe editor Gavin Hewitt says the theme has irritated unions - which are holding their own May Day event - as it implies that they do not understand the value of work.
During the rally, Mr Sarkozy accused the unions of "impoverishing the labour values they claim to defend".
He said the Left had no monopoly on workers' rights and that the 35-hour week - implemented by a previous Socialist administration - had "destroyed hundreds of thousands of jobs".
Meanwhile, Mr Hollande told supporters in the central town of Nevers: "French people want change."
He added that now he was no longer the candidate of the Socialist Party but the candidate of "the whole united Left".
After the first round, far-left candidate Jean-Luc Melenchon - who polled 11% of the vote - told his supporters to rally behind Mr Hollande in the second round.
Mr Hollande has chosen not to attend the unions' rally at the Bastille, which will be addressed by Socialist Party secretary Martine Aubry.
Large numbers of workers and union members are marking May Day with marches and rallies across the country.
Mr Sarkozy continued to court far-right voters on Tuesday in a radio interview, saying France had too many immigrants.
He said: "Our system of integration doesn't work. Why? Because before we were able to integrate those who were received on our territory, others arrived. Having taken in too many people, we paralysed our system of integration."