Italians are voting in a general election, with Prime Minister Silvio Berlusconi facing a strong challenge from centre-left leader Romano Prodi.
Mr Berlusconi's coalition has failed to revive the ailing economy
The polls remain open all day on Sunday and close at 1500 (1300 GMT) on Monday.
Some 50 million Italians are eligible to vote. But opinion polls showed many were undecided.
Mr Berlusconi, a billionaire, has led Italy's longest-serving government since World War II. The economy has proven sluggish for much of his tenure.
The campaign was marked by acrimony, with Mr Berlusconi using vulgar language to describe left-wing voters and Mr Prodi likening him to a drunkard.
Mr Prodi, a former president of the EU Commission, was narrowly ahead in most opinion polls until they were suspended 10 days ago under electoral law.
His mild-mannered - some say lacklustre - style contrasts sharply with Mr Berlusconi's media-savvy flamboyance.
Mr Berlusconi, in power since 2001, cast his ballot in Milan, where he has his main residence and has offered to abolish local property taxes.
He has battled to fend off prosecution for alleged corruption and conflict of interest over his media empire.
He accompanied his 95-year-old mother to the polls. A scrutineer reproved him, half in jest, when he showed his mother where to put a cross against his Forza Italia Party on the ballot paper, the BBC's David Willey reports.
Mr Prodi, who beat Mr Berlusconi in a general election 10 years ago, voted in his home town Bologna.
Both leaders face the tricky task of holding together broad coalitions. That task could be made harder by the new proportional voting system, which is expected to result in a smaller parliamentary majority.
Romano Prodi has struggled to match Mr Berlusconi's flamboyance
Voter turnout was slightly lower than in the last election, with fewer than one in five Italians voting in the first few hours of polling.
Some voters expressed their unhappiness with the personal attacks by the two rivals.
"I'm glad the campaign is over. It was ugly, ugly... and not fair to the electorate," said 63-year-old teacher Edvige Cesarei.
"I've always voted for the centre right, but this time, I don't know."
Others agreed that issues had taken second place to personalities.
"I'm not happy with how the campaign has gone," said Lidia Gentile, 53.
"The politicians didn't aim to explain their plans to the citizens. I am very disgusted. This isn't the right way to go about things."
Mr Berlusconi, Italy's richest man, has struggled to translate his personal success as a tycoon into progress for the economy.
Growth has slowed to an average of 0.6% a year and Mr Berlusconi's support among business leaders has waned.
Voters are choosing members of both upper and lower houses of parliament. Italians living abroad have also been allowed to vote.
There is a chance, depending on which way the regions fall, that the coalition which controls the Senate (upper house) may be different from the one which controls the Chamber of Deputies.
That could result in a fresh election, as a "grand coalition" of left and right would be hard to put together after such a bitter campaign, analysts say.
Initial results are expected by Monday evening. Exit polls will be released soon after voting finishes.