Nicolas Sarkozy and Francois Hollande took the gloves off for the last major rallies of their ferocious battle for the French presidency, three days before their final run-off.

The tone for the last days of campaigning was set by a fierce television debate on Wednesday, in which the right-wing incumbent Sarkozy and Socialist challenger Hollande traded insults without either landing a knock-out blow.

Hollande remains the pollsters' favourite to win on Sunday, but Sarkozy refused to cede any ground, appearing at a huge rally in the southern city of Toulon on Thursday to denounce his opponent as a threat to French values.

"When you want to give immigrants without French citizenship voting rights, that's not the republic," he declared, referring to Hollande's pledge to allow French residents from outside Europe to vote in municipal elections.

"When there are urban ghettos where the law is not respected, that's not the republic. When you wipe out borders, when you don't even dare speak of national identity, that's not the republic," he thundered, to loud applause.

"The left is destroying the republic with its habit of regarding all things as having equal value," he said. "It's time for a national burst of energy."

Hollande was just as determined, in front of a similar huge crowd in another southern city, Toulouse, where he denounced Sarkozy's record in office and predicted a Socialist victory, while cautioning against complacency.

"You will hunt for victory, you will conquer it, tear it from the hands of the right," he declared, his voice hoarse after a long campaign and dozens of stump speeches and television appearances, including Wednesday's debate.

Even as Hollande was speaking, he received a boost from one of the defeated first-round candidates, centrist Francois Bayrou, who revealed he would vote for Hollande despite concerns about his commitment to deficit-reduction.

While Bayrou said he would not instruct the nine percent of the electorate who voted for him in the first round to vote one way or another, he said he had been offended by Sarkozy's lurch to the right since the first round.

"I, personally, will vote for Francois Hollande," he said, expressing regret that the incumbent was pursuing the support of the 18 percent of the electorate that backed the far-right's Marine Le Pen.

Bayrou noted in particular that he had been shocked by a Sarkozy television spot in which his campaign juxtaposed his promise to cut immigration with images of crowds of migrants and a customs post sign with an Arabic inscription.

Bayrou's belated declaration was not expected to change the electoral map. Polls have long forecast that Hollande will win Sunday's run-off by around 54 percent to 46, and they show no signs of shifting before polling day.

Following his second-place finish in the first round, Sarkozy reached out to the 6.5 million voters who had backed Le Pen's far-right anti-immigrant ticket, toughening his rhetoric on national borders and social issues.

Most observers now expect a Hollande victory, after Wednesday's debate proved indecisive, despite fierce exchanges.

Sarkozy had hoped to dominate, but instead Hollande belied his image as a soft consensus-builder by repeatedly attacking the incumbent.

Sarkozy has trailed in opinion polls for more than six months and in the debate the clearly frustrated president called Hollande a "liar" and "arrogant" several times.

Hollande's response was sometimes mocking, accusing Sarkozy of refusing to take responsibility for his record -- and of self-satisfaction in a period of grim economic crisis for many voters.

"Francois Hollande's only weakness compared to Nicolas Sarkozy, that he's viewed as soft and blurry, was overcome last night," said Gael Sliman of the BVA opinion poll institute.

A total of 17.79 million people watched the almost three-hour duel, audience monitor Mediametrie said, fewer than the 20.4 million who watched Hollande's former partner, Socialist Segolene Royal, take on Sarkozy in 2007.

An LH2-Yahoo poll said 45 percent of those who watched the debate found Hollande more convincing, 41 preferred Sarkozy and 14 percent had no opinion.

Sarkozy meanwhile dismissed reports that Moamer Kadhafi's regime had funded his 2007 election campaign.

French news website Mediapart on Saturday published a 2006 document it said showed Libyan backing.

On Thursday in Tunisia, lawyers for Libya's former premier, Baghdadi al-Mahmudi, said the Kadhafi regime had backed Sarkozy to the tune of 50 million euros ($65 million). Mahmudi is currently fighting extradition from Tunisia to Libya.

But Libya's National Transitional Council (NTC), successors of Kadhafi's ousted regime, said Thursday they thought the note was a fake.

Khadafi's former intelligence chief and foreign minister Mussa Kussa, allegedly involved in the transaction, has also dismissed the document as forged.

Sarkozy has denounced the affair as a bid to disrupt his re-election campaign and is suing the Mediapart website.