Podgorica: Montenegro said it had foiled a plot by a group of Serbs who were planning to seize the prime minister and parliament and proclaim victory for the opposition after Sunday`s tinderbox election.
News of the arrest of 20 Serbs ratcheted up tensions in the small ex-Yugoslav state which were already high over veteran premier Milo Djukanovic`s plans to forge closer ties with the EU and NATO.
The pro-Russian opposition -- which opposes his policies -- branded the announcement of the arrests as propaganda, while Serbia sardonically questioned its timing.
"We suspect that this criminal group was aiming to arrest the prime minister of Montenegro," the prosecutor`s office said in a statement.
"The plan was to attack citizens gathered outside parliament, as well as police, during the announcement of the results, and then take control of parliament and proclaim victory for the opposition," it said.
Montenegro police chief Slavko Stojanovic had said earlier that the suspects were allegedly planning attacks on organs of state, the police, and senior state officials, adding that a 21st suspect was being hunted.
Andrija Mandic, head of the Democratic Front, Montenegro`s main opposition, immediately condemned the announcement as "gross propaganda".
And in neighbouring Serbia, Prime Minister Aleksandar Vucic questioned the timing of the disclosure of the alleged plot.
"I find it curious that this is happening today, and that`s all I`ll say," Vucic said, according to the Tanjug news agency. "As for the rest, it would be better for me to bite my tongue."
The alleged plot was revealed as the country`s half a million strong electorate voted in a poll pitched by Djukanovic as a choice between closer ties with the West or with longtime ally Russia.
Polls closed at 1800 GMT with authorities putting turnout at 73.2 percent -- a high figure for the region. The first results were expected later Sunday night.
Mobile messaging services including WhatsApp and Viber were briefly down late Sunday at the behest of the authorities.
Interior Minister Goran Danilovic, who belongs to an opposition party, meanwhile urged voters not to head into the streets to celebrate.
Djukanovic, 54, is the only Balkan leader to have held on to power since the collapse of Yugoslavia began in the early 1990s, serving several times as prime minister and once as president in the country.
But analysts say he is now under pressure, with critics accusing his government of cronyism, corruption and links to organised crime.
The latest internal party polls forecast his Democratic Party of Socialists (DPS) leading with less than 40 percent of the vote, meaning coalition partners would be needed to form a government.
"Even if the DPS could reach with their political allies some tiny majority, that would be unstable," said Zlatko Vujovic, director of the Centre for Monitoring and Research, a watchdog group.
One of the six founding republics of the former Yugoslavia, Montenegro was joined in a loose union with Serbia after the Yugoslav breakup.
The union ended in 2006, when the country narrowly voted in favour of independence, and relations with Serbia have been fraught ever since.
Barricades were put up near parliament, apparently to protect the building against any post-election violence.
Opposition leader Mandic urged his supporters to stay calm.
"Everything has been set in place to defend Milo Djukanovic`s government," he said. "Tonight, massive chaos is expected -- but only in the prime minister`s office, when the results are announced."Djukanovic, who faced large anti-government rallies last year, has accused Russia of funding opposition parties.
"Are we going to be part of developed European society or a Russian colony?" he asked supporters waving national red flags at his final rally in the capital.
Montenegro was invited to join NATO in December, and ratification of the deal will be put to the next parliament.
But the issue profoundly divides the country`s 620,000 people, prompting reminders of the bonds with Russia and the alliance`s 1999 bombing of Yugoslavia.
Moscow, already angered by EU sanctions against it over the Ukraine conflict, has warned of consequences if the Adriatic republic joins the Atlantic alliance.
The Democratic Front opposes membership of either the EU or NATO, and is demanding a referendum on joining the alliance. It staged violent protests on the issue in 2015.
Other opposition groups have more mixed positions -- some are pro-EU but would also like a referendum on NATO -- yet they have spoken of joining forces to oust Djukanovic.