Vujanovic is convinced of his victory
A pro-independence candidate in the former Yugoslav republic of Montenegro, Filip Vujanovic, is heading towards an overwhelming victory, according to unofficial results.
"We can say with certainty that Filip Vujanovic has become president with more than 65% of the vote cast," Zlatko Vujovic from independent observer group Centre for Monitoring (Cemi) was quoted as saying by the AFP news agency.
Mr Vujanovic is the speaker of Montenegro's parliament and a member of the governing Democratic Party of Socialists.
Correspondents said his two rivals - Miodrag Zivkovic, the leader of the Liberal Alliance, and the independent candidate, Dragan Hajdukovic - did not have enough public support.
Mr Vujanovic won presidential elections in December and February, but because less than half the electorate turned out to vote the results were cancelled on both occasions.
But that law has been amended in favour of a simple majority of the votes cast.
Cemi said that about 48% of some 456,000 eligible voters had cast their ballots on Sunday.
First official results are expected to be announced on Monday.
Correspondents say opposition parties remain deeply divided and failed to put forward their own joint candidate.
Montenegro has been without a president since November when the then incumbent, Milo Djukanovic, became prime minister - a post holding more power than president.
Mr Djukanovic and Mr Vujanovic are close allies.
One of the key tasks of the new president will be to oversee the development of the new state of Serbia and Montenegro which came into being in February out of the ashes of the old Yugoslavia.
Each republic in the loose union has the right to hold an independence referendum after three years.