Nicolas Sarkozy and Francois Hollande are making their final pitch for votes in one of the most dramatic elections in recent French history.
Opinion polls suggest the incumbent president has cut Mr Hollande's lead slightly but is still trailing his Socialist challenger by some 6%.
Analysts say he would need a major reversal in fortunes to win now.
He suffered a blow when centrist Francois Bayrou, who took 9.1% in the first round, backed Mr Hollande.
In a radio interview on Friday, Mr Sarkozy said he would form a new government in 48 hours if re-elected.
He is also set to give a speech at Sables d'Olonne in western France.
Mr Hollande will travel to Moselle for a campaign speech, followed by a visit to Perigueux in the Dordogne region for his final rally.
Campaigning officially finishes at midnight local time (22:00 GMT).
'Beaten long ago'
Mr Bayrou came fifth in the first round of voting.
Though he once served with the president in a right-wing cabinet, he says Mr Sarkozy has gone too far to the right.
He said he was "not making any voting recommendation", but accused Mr Sarkozy of abandoning European values and moving too close to the far-right position on immigration.
The BBC's Hugh Schofield in Paris says that unless the polls turn out to be wrong, Mr Sarkozy looks set to be the first French president since Valery Giscard d'Estaing in 1981 not to win a second term.
The president has been courting the votes of the supporters of far-right leader Marine Le Pen, but she has already said she will be leaving her ballot blank rather than vote for Mr Sarkozy or his rival.
Ms Le Pen, who attracted 6.4 million votes in the first round, said on Thursday that the election was over as Mr Sarkozy was "beaten a long time ago".
The two rival candidates attended final mass rallies on Thursday in friendly territory: Mr Hollande in the south-western city of Toulouse and Mr Sarkozy in the southern port of Toulon.
The rallies follow a heated televised debate on Wednesday night, watched by an estimated 17.9 million people.