By Philip Pullella
ROME (Reuters) - Several million Italians voted in a run-off primary election on Sunday to choose the center-left candidate for prime minister in next spring's national elections.
The contest will decide whether Pier Luigi Bersani, 61, or Matteo Renzi, 37, stands for the Democratic Party (PD) against a still-to-be-chosen center-right candidate to take over from Prime Minister Mario Monti.
Thousands of voting stations, many of them white canvas gazebos on street corners, will close at 8 p.m. (1900 GMT) and exit polls will be released shortly afterwards.
U.S.-style party primaries are relatively new to Italy and have been welcomed by many as a departure from the past, when the party hierarchy chose the candidate for prime minister.
Most opinion polls say the slow-speaking, bald, professorial Bersani will defeat Renzi, who bounces around platforms at rallies in open shirts and jeans.
More than three million supporters of the PD voted in the first round a week ago -- which eliminated three other candidates -- and a similar number was expected to turn out for the final vote. Some people queued for hours.
Some 100,000 people who did not vote last week asked to vote on Sunday but only some 7,000 requests were accepted because they had what party rules considered legitimate reasons for having missed the first round, such as health problems.
While markets are wary of Bersani's alliance with a party called Left, Ecology and Freedom, both men have pledged to continue budget discipline started by Monti. They would put more emphasis on economic growth and easing burdens on workers and pensioners.
Bersani, who says he represents experience, won 44.9 percent of the vote in a first-round last Sunday. Renzi, who paints himself as a Kennedy-esque reformer and insists the Democratic Party (PD) needs a big shake-up, took 35.5 percent. Three other candidates divided the remaining votes.
A poll by the SWG organization said Bersani, who is PD leader, would get 53-57 percent in the run-off and Renzi, mayor of Florence, 43-47 percent.
"I don't ask you to like me. I ask you to believe me," Bersani told supporters at a rally on Saturday night, repeating in his stump speech that a steady, experienced hand was what Italy needed in tough financial times.
The past week has seen bitter argument between the two candidates over whether Renzi violated contest rules by taking out privately-funded advertising urging those who did not participate in the first round to vote for him in the run-off.
Bersani tried to put the spat behind them, saying the party did not need to inflict "friendly fire" on itself.
Renzi touted his determination to be a reformer.
"As mayor of Florence, I cut costs, I eliminated office cars for city employees," Renzi told a rally on Saturday night in his trademark style, wandering across the stage with a long-lead microphone.
"As prime minister, I will do the rest," he said.
Renzi accused the older generation of the Democratic Party of failing to present a credible alternative, allowing former Prime Minister Silvio Berlusconi's center right to govern for so long.
"If the other side wins, nothing will change. If we win tomorrow night, there will be a new Italy," he said.
Monti, favorite of the business community, has insisted that he will not be a candidate next year but has said he will come back if the election does not provide a clear winner.
Another possible future role for him is as president of the republic and guarantor that austerity reforms agreed with Italy's European partners continue.
Italy's gross public debt is equivalent to 126 percent of national output, according to the IMF.
Berlusconi's scandal-plagued right, forced from government by the financial crisis a year ago, is in disarray.
Berlusconi said on Monday he would wait to see who wins the center-left primary before deciding whether to run himself. He has repeatedly changed his mind in the last few weeks on whether to do so.
(Editing by Roger Atwood)