* Democratic Party leader Bersani seen as frontrunner
* Florence mayor Renzi to be main challenger
* Vote result expected late on Sunday (Adds comments from Bersani, Renzi)
Italy's centre left voted on Sunday to choose the candidate who will be the leading contender to succeed Mario Monti as prime minister after a election in March and take charge of steering the country through a deep recession.
Opinion surveys show Democratic Party leader Pier Luigi Bersani is the frontrunner among five candidates, followed by Florence mayor Matteo Renzi, who has vowed to shake up Italy's political establishment if he is chosen.
The vote will eliminate a major element of uncertainty before the election to choose a successor to Monti's technocrat government.
The centre-left alliance is well ahead in opinion polls for the parliamentary election and the winner of the primary vote is in pole position to take over Monti's efforts to control strained public finances and tackle a year-long recession.
Support for Prime Minister Silvio Berlusconi's deeply divided centre-right People of Freedom party (PDL) has crumbled to less than half than it recorded in the last election in 2008.
Berlusconi said on Saturday he was again thinking about running, deepening the PDL chaos.
Both Bersani and Renzi reject the idea, encouraged by international markets, that Monti should return after the vote to continue his economic policies that have so far included unpopular spending cuts, tax rises and labour reform.
Protests on Saturday by tens of thousands of students and workers from across the political spectrum highlighted the levels of discontent among Italians grappling with the slump and rising unemployment in the euro zone's third biggest economy.
While Renzi, 37, is much more popular across the general population than career politician Bersani, 61, he is far weaker among party supporters who will decide the primary.
About 3 million party and non-party voters are expected to take part in the poll that is open to anyone over 18 who is Italian or resident in Italy regularly, but it is likely that members will make up the biggest proportion of those at the ballot box.
However, Bersani may still fail to secure the 50 percent he needs for a first-round victory, which will mean a second round run-off will be held on Dec. 2. At that point he is likely to pick up the votes of third-placed Nichi Vendola, the openly gay governor of the southern Puglia region.
"I am expecting a run-off because there are a lot of us," said Bersani, talking to reporters at a voting station on Saturday. "All five of us are going to try to lend a hand to help this country emerge from its suffering."
An upbeat Renzi told reporters before joining in a half-marathon in Florence: "If I lose it will have been a great people's experience, but I'm going to win."
The centre right is due to hold its own primaries on Dec. 16, but PDL secretary Angelino Alfano said on Saturday that would no longer make sense if Berlusconi chose to stand.
Further complicating the national political picture is the dramatic rise of the anti-establishment 5-Star Movement, which is now second in opinion polls, and that around half of Italians say they are either undecided or will abstain.
It is also still unclear what electoral system will be used for the national vote, expected on March 10-11, as politicians have been arguing over how to reform an unpopular electoral law that allows party leaders to hand-pick members of parliament.
Voting on Sunday was due to end at 8 p.m. (1900 GMT), with results due around midnight (2300 GMT).