Kerviel, 37, said he was appealing to French President Francois Hollande to intervene in his case. He has spent over three years fighting charges stemming from massive market bets that almost brought French bank Societe Generale to the brink of collapse in 2008.

He has been convicted of breach of trust, forgery and fraudulent data manipulation. He embarked on a march on foot from Rome to Paris earlier this year, meeting the pope and portraying his case as one individual's struggle against high finance.

French judges have required him to report to a French police station by Sunday evening to start serving his sentence, which was confirmed by France's highest appeals court in March.

"I am staying here for now, waiting for the President to respond," Kerviel told reporters in the Italian town of Ventimiglia, close to the French border. He said he was not guilty and that his criminal conviction was unfair.

French prosecutors said that if Kerviel did not show up at the southern French police station of Menton by midnight, he would be considered as on the run and France would issue a European arrest warrant to ensure he serves his sentence.

The warrant would be issued this week and would require Italian authorities to hand him over to France, a judicial source said.

Speaking to a crowd of TV reporters following him throughout the weekend, Kerviel called for President Hollande to give immunity to the unnamed witnesses he said would be willing to testify in his favour.

Kerviel's lawyer, David Koubbi, said he would be returning to Paris later in the day in the hope of meeting the president.

Kerviel has never denied masking his 50-billion-euro (40.7 billion pounds) positions but has accused his bosses of knowing what he was doing. The courts, however, have each time backed SocGen's insistence that the trader acted alone.

France's highest appeal court upheld his three-year jail term in March but it quashed the 4.9-billion-euro civil damages designed to compensate SocGen for its losses when it unwound the trader's mammoth positions in the midst of the financial crisis.

The French president's office said in a statement on Saturday evening that if Kerviel asked for presidential grace, his request would be examined according to usual procedures.

Kerviel rebuffed the offer: "Asking for grace is admitting you're guilty, something I've been fighting for six years. I'm not guilty. And I will never get down on my knees in front of such an unfair ruling," he said.

In an emailed statement on Saturday, a spokesman for Societe Generale criticized the media hype around Kerviel, stressing he had been subject to a meticulous investigation, judged three times and found guilty each time.

(Reporting by Chine Labbe and Natalie Huet; Editing by Stephen Powell)