Segolene Royal accused her rivals of clinging to "outdated" ways
France's opposition Socialists are voting for a new leader, amid bitter infighting and splits in the party.
Former presidential candidate Segolene Royal is up against Lille Mayor Martine Aubry and leftist MEP Benoit Hamon.
The leadership issue is being decided by 233,000 party members in Paris. The winning candidate needs at least 50% of the vote to avoid a run-off on Friday.
The voting comes after last weekend's party congress - that was meant to back a single candidate - ended in disarray.
The party has been beset by infighting since France's last Socialist President Francois Mitterrand stepped down in 1995.
The three candidates are vying to replace outgoing chief Francois Hollande.
At their annual congress in Reims at the weekend, top party members failed to reach consensus on a new leader.
Ms Royal - who had won a pre-convention ballot with 29% of members' votes - accused her rivals of clinging to "outdated" ways after they refused to rally behind her leadership bid.
One of the candidates, Paris Mayor Bertrand Delanoe, even pulled out of the race to "avoid creating further confusion".
"The Socialist Party is gravely ill," Mr Delanoe said.
Ms Royal, defeated by Mr Sarkozy in last year's presidential elections, has been accused seeking to transform the party into her own personal electoral machine, the BBC's Emma Jane Kirby in Paris says.
Martine Aubry came up with the 35-hour week
Ms Aubry, the architect of France's 35-hour week, is seen by many as being too old-fashioned, although she has the backing of Mr Delanoe, our correspondent says.
She adds that Mr Hamon is seen as a rising star in the party, but he has been criticised for being too far-left.
France's media has said the Socialists showed themselves at the Reims convention to be "ungovernable".
In a scathing editorial, France's left-wing Liberation newspaper said that now Mr Sarkozy knew he was dealing with a party "without a clear political line, without a strategy, without an uncontested leader".
Meanwhile, the right-wing Figaro said the Socialists "have gone mad".
It is clear that whoever becomes the new leader of the opposition will have a tough job uniting party members to build a credible challenge to President Sarkozy's government, our correspondent says.