By Brian Love

PARIS (Reuters) - Moderate left-wing candidate Francois Hollande led in a primary vote for the French Socialist Party's presidential nomination Sunday, making him the favorite to unseat President Nicolas Sarkozy in April.

More than 1.5 million people voted nationwide in France's first U.S.-style primary election to select a presidential candidate, which the Socialists say was inspired by the process that gave Barack Obama the momentum to take the White House.

The Socialist Party published estimates showing Hollande in front with 39 percent of votes, short of an absolute majority and thus headed for a runoff on October 16.

He is expected to face Martine Aubry, a one-time labor minister and daughter of former European Commission president Jacques Delors. She won 31 percent in the preliminary readout.

Opinion polls have predicted that Hollande, a witty if unexciting party veteran who has never been a government minister, would not only win the Socialist primary but would defeat Sarkozy by a comfortable margin if the two face off in the presidential battle next April.

That would make him the first Socialist leader in the Elysee Palace for 17 years.

Socialist Party chief Harlem Desir described the turnout on a drizzly day across much of the country as a triumph for democracy, the party and its future presidential candidate.

Previous primaries were closed-shop events, limited to some 200,000 party members in a country of more than 65 million. This

time, any registered voter who paid a euro ($1.35) and professed to support left-wing values could participate.

"It's not expensive to get rid of Sarkozy," Hollande said before Sunday's ballot, where he competed with five others including Aubry and Segolene Royal, his former partner and mother of his four children.

The Socialists were deprived of a potentially strong candidate this year when Dominique Strauss-Kahn resigned his post as head of the International Monetary Fund after being charged with a sex assault in New York. The charges were later dropped, but Strauss-Kahn's political career has not recovered.

Hollande's closest rival has always been Aubry, minister from 1997 to 2002 when a left-wing government last held office.

In a mini-surprise, the preliminary results showed anti-globalization contender Arnaud Montebourg in third place with about 17 percent of votes. Royal, who lost the 2007 presidential election to Sarkozy, had been expected to place third but sank to a score of seven percent.

VICTORY BY NUMBERS

The novelty of the primary contest may give the Socialists a boost if, in addition to the healthy turnout, it produces a clear-cut winner, political analysts say.

Millions have tuned into live television debates between the Socialist candidates in the past three weeks as the primary appeared to have captured the popular imagination.

Sarkozy's UMP party, which criticized the Socialists' motives for the innovation at the outset, are considering doing the same, not in time for the upcoming presidential election but the one after it, according to Prime Minister Francois Fillon.

In opinion polls, Sarkozy remains deeply unpopular and voters are ready to see the left take power. Parliamentary elections will be held on the heels of the presidential election, with a first ballot on April 22 and run-off on May 6.

The frontrunners in Sunday's contest have sparred regularly but gently about policy alternatives.

Their party manifesto established the basic thrust of their policy, which is to repeal 50 billion euros of tax breaks introduced under Sarkozy, using half of that money to cut the deficit and the other half to promote jobs and economic growth.

(Additional reporting by Marie Maitre, Elizabeth Pineau and Sophie Louet; Editing by Peter Graff)