Romanians are voting in a run-off election between Prime Minister Adrian Nastase and opposition candidate Traian Basescu, the mayor of Bucharest.
Voting was said to be slow in the first few hours
Neither won the required 50% of the vote in the first round on 28 November, which was marred by fraud allegations.
Tighter controls have been introduced ahead of the poll and more than 3,000 observers will monitor the vote.
Mr Nastase received an election boost after concluding European Union accession talks on Wednesday.
Mr Nastase, a Social Democrat, led in the first round with 41% of the vote, ahead of centre-right opposition candidate Mr Basescu, who got 34%.
Campaigning ended with Mr Basescu considered the winner of the only face-to-face debate broadcast over the past weeks.
But Mr Nastase still leads in opinion polls and according to analysts only a massive turnout of urban voters, more favourable to the opposition, could reverse this trend.
Polls opened at 0700 (0500GMT) and will close at 2100 (1900GMT), with up to 18 million people eligible to vote.
But voting appeared slow for the first few hours, as temperatures were below freezing.
Fears remain of irregularities such as multiple voting, with police looking out for voters being bussed between polling stations, AFP news agency reported.
And the country's main civil rights organisation, Pro Democratia, said it would try to make up for what it described as official unwillingness to observe regulations.
"Today's vote will answer Romanians' concerns for stability and prosperity tomorrow," Mr Nastase said after voting at a Bucharest school. "With God, let's move forward."
His rival sought to exploit dissatisfaction with politics.
"Today Romanians choose between continuing with corruption, disregard for people and giving Romania back to Romanians," Mr Basescu said at a polling station in central Bucharest.
Endemic corruption in Romania and the government's hopes for the eastern European state to join the EU in 2007 dominated the campaign.
Mr Nastase's prospects will have been strengthened after his government finished EU accession talks this week.
Under the deal, Romania's membership, scheduled for January 2007, could be put off for a year if it does not fulfil commitments on reform, root out corruption and strengthen its eastern border security.
The opposition and several non-governmental organisations have said the first round was marred by "massive fraud".
The electoral commission, which came under fire for "covering up irregularities" in the first round, has agreed to enforce stricter rules in order to prevent people voting more than once.
The Social Democrats say there were only minor irregularities, and accuse Mr Basescu of damaging Romania's image abroad with the allegations.