France's National Front leader Marine Le Pen has vowed to cast a blank vote in Sunday's presidential poll run-off.
Ms Le Pen told a mass rally in Paris she could back neither President Nicolas Sarkozy nor the Socialist candidate Francois Hollande and told supporters to follow their conscience.
Ms Le Pen won 6.5 million votes - 17.9% - in the first round of the election.
The latest opinion polls suggest Mr Hollande has a six to 10 point lead over Mr Sarkozy.
Mr Hollande and Mr Sarkozy will go head-to-head in the sole televised election debate on Wednesday.
'Centre of gravity'
On Tuesday, Marine Le Pen led her National Front party's annual rally to its climax at the Place de l'Opera in the French capital.
She was addressing supporters after winning a record number of votes for her party in the first round of the presidential election and after taking over from her father, Jean-Marie Le Pen, just over a year ago.
"On Sunday, I will vote blank," she told the rally. "I have made my choice. Each of you will make yours."
Ms Le Pen praised the campaign her party had run, saying it had touched the spirit of the French people.
"We have become the centre of gravity for French politics," she said.
Ms Le Pen said a "great project of emancipation" had begun and nothing would be the same again.
She rounded on both Mr Hollande and Mr Sarkozy.
Ms Le Pen said Mr Sarkozy's recent policy switches had contradicted his actions over his five-year term and he was not fit to be president.
Mr Sarkozy is holding a large rally on Tuesday in Trocadero Square, Paris, which he says is a showcase of "real work".
BBC Europe editor Gavin Hewitt says this has irritated unions as it carries the implication that the Left do not understand the value of work.
Mr Hollande has said Mr Sarkozy is more the president of "real unemployment".
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 an interview on the RMC radio station, 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."
Source: BBC News