Tensions went up by a notch during elections in Montenegro Sunday when police said they had arrested 20 Serbian nationals, accusing them of intent to destabilize the country by terrorism.
“These persons are suspected ... of acts to create a criminal enterprise and terrorism,” national police director Slavko Stojanovic said in a statement.
“They are suspected of intent to first take automatic weapons, then to execute attacks on institutions of the systems, police and representatives of state bodies, not excluding high state officials,” he said.
Before the elections, pro-Western Prime Minister Milo Djukanovic - who has been in power since 1991 - accused Serbia of serving Moscow's interests in an attempt to put Montenegro's drive to join NATO and the European Union into reverse.
In Belgrade, Serbian Prime Minister Aleksandar Vucic said he had no information about the incident and refrained from commenting the incident until Monday, in order not to influence the outcome of elections in Montenegro.
The arrest happened overnight, but the statement was released as Montenegrins were voting in polls that will decide the question of whether Djukanovic and his Democratic Party of Socialists (DPS) will continue their 25-year long reign.
Interior Minister Goran Danilovic urged all political parties not to call for street celebrations and the people to remain in their homes.
Montenegrins had been showing up at a fast pace throughout the day: with three hours left until the 8 pm (18000 GMT) close, 55.9 per cent had cast their ballots, 5.3 per cent more than in 2012 polls, the Centre for Democratic Transition said.
The final turnout four years ago reached 70.3 per cent among the 530,000 registered voters.
There were no credible surveys ahead of the elections, but the race was expected to be closer than ever since Djukanovic and the DPS ascended to power a quarter century ago.
After being too fragmented to seriously challenge Djukanovic and the DPS in the 10 elections since the first multi-party elections 25 years ago, the opposition now appears united around the single goal of toppling him.
Local observers estimate that the higher turnout favours the challengers, reflecting the interest of previously lethargic abstainers to become involved and influence the outcome.
Montenegrins remain polarized over many issues.
They argue about allegiances between the county's Orthodox "mother," Russia, and its "sister," Serbia. Other debates focus on NATO, whether they should have their own church or remain under the Serbian patriarch, or even which language they speak: Serbian or Montenegrin.
The campaign ahead of Sunday's vote, however, largely focused on Djukanovic, with his camp insisting that a vote for him is a vote to continue the legacy of independence claimed a decade ago and to remain on course to NATO and EU membership.
NATO invited the former Yugoslav republic in late 2015 to join. Podogorica has also pushed ahead with EU membership talks.
The opposition, which is not unified regarding key issues such as NATO membership, accuses Djukanovic with a single voice on the topic corruption, likening his reign to turning Montenegro into a private business. Pollsters predict a very close race.
Polling stations close at 8 pm (1800 GMT) and first projections are expected within a few hours.