The parliamentary vote is marked by a dispute over a law on religious rights that is staunchly opposed by the influential Serbian Orthodox Church.
The issue has fueled divisions in the nation of 620,000 people that has defied its traditional Slavic allies to become independent in 2006 and join NATO in 2017.
Indicating high interest in the election, the turnout in the first hours of balloting was about 14 per cent higher than the same time four years ago.
Months of church-led protests against the property bill have raised tensions and fears of potential incidents during and after the election on Sunday.
Authorities are also thinking back to the previous election, in October 2016, when they said they thwarted a planned election-day coup orchestrated by two Russia military intelligence officers.
Prime Minister Dusko Markovic said the state will deal with any attempts to affect this election.
“This is the day when Montenegro decides to move strongly forward toward economic and general development — a Montenegro that is a member of the the European Union and a reliable member of NATO,” he said.
Some 540,000 voters are choosing whether to keep in power the Democratic Party of Socialists, which has governed Montenegro for some 30 years.
The party led Montenegro to independence peacefully from much larger Serbia and into NATO, despite strong opposition from Russia.
However, the DPS and its leader, Montenegrin President Milo Djukanovic, have faced accusations of an autocratic rule, widespread graft and criminal links.
Djukanovic has said Sunday’s vote will determine whether Montenegro will continue toward membership in the European Union or allow Serbia and Russia to install their stooges.
“I am absolutely convinced that the democratic will of the majority is on the side of Montenegro and its European future,” Djukanovic said after voting on Sunday. He cited alleged attempts to “induce” tensions from outside Montenegro and blasted neighboring Serbia.
“We have all together registered that stampede …. in which the entire Serbian media and political scene is involved,” said Djukanovic. “We could possibly pick with tweezers someone who is not involved.”
The Montenegrin president, known as the longest-serving European leader, has been a key Western ally in the efforts to push the volatile Balkan region toward Euro Atlantic integration.
Opinion polls ahead of the election have predicted that the DPS will finish ahead of other groups, but might not garner enough votes to form the government on its own.
The main opposition group, the pro-Serb and pro-Russian “For the future of Montenegro” alliance, has backed church-led protests against the religion law, and it wants closer ties with Belgrade and Moscow.
The Serbian Orthodox Church has argued that the law allows the Montenegrin state to confiscate its property as a prelude to setting up a separate Montenegrin church. This has been denied by the government.
About one third of Montenegro’s 620,000 people declare themselves as Serbs, which makes relations with Serbia and the Serbian Orthodox Church a highly sensitive issue.
Third-placed in pre-election surveys has been the “Peace is our nation” group that comprises more moderate parties seeking middle ground in the Montenegrin political dispute.
Several other smaller parties and those run by ethnic minorities are also in the race that is being held amid the new coronavirus outbreak.
The virus this summer ravaged Montenegro’s tourism, which normally feeds the country’s weak economy. The mountainous Adriatic Sea nation is blessed by stunning nature and golden beaches.