Montenegro's Prime Minister Milo Djukanovic, speaks to journalists after casting his ballot at the polling station in Podgorica, Montenegro, Sunday, Oct. 16, 2016. Montenegrins voted Sunday in a parliamentary election that could determine whether the small Balkan state continues on its Western course or turns back to traditional ally Russia. (AP Photo/Darko Vojinovic)

Montenegro's Prime Minister Milo Djukanovic, speaks to journalists after casting his ballot at the polling station in Podgorica, Montenegro, Sunday, Oct. 16, 2016. Montenegrins voted Sunday in a parliamentary election that could determine whether the small Balkan state continues on its Western course or turns back to traditional ally Russia. (AP Photo/Darko Vojinovic)

PODGORICA, Montenegro (AP) — Montenegro’s pro-West ruling party has won the most votes in the small Balkan country’s parliamentary election on Sunday, according to unofficial results, but without enough support to govern alone and assure the country’s chartered course into NATO and other Western institutions.

The tense election was marked by the arrest of 20 people suspected of planning politically motivated armed attacks after the polls closed in the vote during which Prime Minister Milo Djukanovic faced the toughest challenge yet to his quarter-century rule.

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The independent CeMI election monitoring group said that with 100 percent of the vote counted, the Democratic Party of Socialists won 41 percent, while two opposition parties — the Democratic Front and the Key Coalition have — 20 and 11 percent respectively. The monitoring group used its own vote count at the polling stations.

Both the ruling party and the opposition claimed they have enough seats to form the country’s future government. They will have tough time to form post-electoral coalitions with small groups that enter the 81-seat parliament. The outcome of those negotiations will determine whether the state continues on its Western course or turns back to traditional ally Russia.

Partial official results are expected to be released Monday and complete results on Tuesday.

Djukanovic told his supporters that he will soon start negotiations with potential coalition partners.

“A big power has moved against the small Montenegro, but we have succeeded,” Djukanovic said, referring to Russia and its open support of the opposition.

“Tonight we can safely say that Montenegro is heading toward the European integrations and we will soon ratify the accession agreement with NATO.”

The parliamentary vote pitted Djukanovic’s Democratic Party of Socialists against a cluster of pro-Russian and pro-Serbian opposition groups that staunchly oppose the government’s pro-Western policies, especially its NATO bid.

Police Director Slavko Stojanovic said those arrested Saturday night came from Serbia and planned to collect automatic weapons to attack state institutions, police and possibly state officials after the vote.

Serbia’s former special forces commander, Bratislav Dikic, who was removed from the post in 2013 because of his alleged criminal activities, was among those arrested.

Stojanovic said they were charged of “forming a criminal organization and terrorism.” He said one Serbian is still on the run.

The prosecutor’s office said the group planned to attack people who gather in front of the parliament when the vote results are proclaimed, then storm the building in the capital and declare the victory “of certain parties” in the election. The statement said they also planned to arrest Djukanovic.

Police vans were seen bringing in the handcuffed suspects to the prosecutor’s office in Podgorica, the capital.

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There have been fears that violence could erupt on the streets of Podgorica between opposition and government supporters after the results of Sunday’s vote were announced. The government also said hackers have attacked several web sites, including that of the ruling party.

Montenegro’s interior minister has warned people to stay indoors instead of celebrating in the streets after election results are announced. Police vehicles were seen on the outskirts of the capital, but the streets were quiet.

Serbian Prime Minister Aleksandar Vucic, meanwhile, hinted that the arrests might have been staged by the government.

“It’s a strange day on which all this is happening, that is all I will say,” Vucic said.

The scenic country of 650,000 people, squeezed between the Adriatic Sea and towering mountains, is deeply divided among those who favor and those who oppose Western integration. The outcome of Sunday’s vote could jeopardize NATO and European Union enlargement in southeastern Europe and could prove decisive in the Kremlin’s attempts to regain influence in the strategic Balkans region.

Predominantly Orthodox Christian like Russia, Montenegro was Moscow’s historical ally. But after splitting with Serbia in a 2006 referendum, Montenegro took a strong turn toward Western integrations.

Russia strongly opposes the expansion of NATO in European ex-communist countries it considers part of its “strategic interests.” The West is wary of Russian influence in the still-volatile region, which was engulfed in bloody civil wars in the 1990s.

Opposition leaders have accused Djukanovic of corruption, nepotism and economic mismanagement and say he is trying to scare voters by suggesting that chaos will prevail if he loses.

“The only chaos will be within Djukanovic’s cabinet,” opposition Democratic Front leader Andrija Mandic said after he voted Sunday.

“I have no doubt that the opposition will show its strength and that the Democratic Front will become future framework of the Montenegro government,” he added. “Today is the last day of Djukanovic’s 27-year rule.”

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Associated Press writer Jovana Gec contributed from Belgrade, Serbia.