PARIS - French Prime Minister Dominique de Villepin underwent more than 17 hours of questioning by judges investigating a suspected smear campaign against a political rival, a marathon grilling that spilled into early Friday.

The scandal centered on damaging - but false - allegations that Nicolas Sarkozy, France's interior minister who is widely considered a leading presidential contender, had secret bank accounts.

The affair helped poison relations between Sarkozy and Villepin, unsettling the center-right government in which they form an uneasy pairing, and has played into the race for the French presidency in 2007.

Key questions are what Villepin knew, and when, and whether he kept an investigation into the allegations going long after it became clear that Sarkozy had been unjustly accused. Judges were questioning Villepin only as a witness.

The questioning began at 9 a.m. Thursday and lasted until nearly 3 a.m.

"For my part, I was very pleased to be able to testify on this matter, of which I have for many months been a victim of slander and lies," Villepin told reporters after emerging from what he called his "marathon testimony."

Suspicions that Villepin was involved in the accusations - combined with his shelving of proposed labor reforms, which provoked violent protests last spring - appear to have dashed his hopes of running for president.

Sarkozy, on the other hand, widely seen as a victim of the affair, has emerged from it strengthened, leaving him the overwhelming favorite to win the presidential nomination for the center-right UMP party, which he heads.

His supporters have called for punishment if the investigation establishes that the apparent smear campaign was aimed at unsettling the Serkozy's presidential run.

"When low blows are used to sideline a political adversary, there must be sanctions," Francois Fillon, Sarkozy's political adviser, said in a radio interview. He said he believed that "people manipulated this affair" but added "I have no proof."

Other prominent center-right UMP party politicians have been questioned as witnesses, including Defense Minister Michele Alliot-Marie and former Prime Minister Jean-Pierre Raffarin.

Jean-Louis Gergorin, a former vice president of European defense giant EADS, and ex-EADS executive, Imad Lahoud, have been charged together with Denis Robert, author of a book that implicates Villepin in the smear campaign.