Montenegro's Prime Minister and long-ruling Democratic Party of Socialists leader Milo Djukanovic, center, speaks in his party headquarters, in Podgorica, Montenegro, early Monday, Oct. 17, 2016. 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. (AP Photo/Darko Vojinovic)

PODGORICA, Montenegro - Montenegro's opposition claimed on Monday that an inconclusive parliamentary election was packed with irregularities, including the authorities' blocking of popular messaging services during the vote day.

Prime Minister Milo Djukanovic's long-ruling party won the most votes in the Sunday ballot, but without enough support to govern alone. Both the opposition and the Democratic Party of Socialists will now have to try form a governing coalition with several small groups represented in the 81-seat parliament.

The outcome of the coalition negotiations will determine whether the state continues on its Western course or turns back to traditional ally Russia.

The tense election was marked by the arrest of 20 people suspected of planning politically motivated armed attacks against Djukanovic and his supporters. Opposition leaders claim that thousands of their supporters were rounded up by the police on election day.

The authorities blocked popular messaging applications Viber and WhatsApp for hours on Sunday, saying "unlawful marketing" was spread through the mobile networks on the election day.

"Blocking such apps is unthinkable in any normal country. I have never heard of that happening anywhere ever in an election," said an opposition party leader, Ranko Krivokapic, who has monitored elections for the Organization for Security and Cooperation in Europe in the past.

The OSCE vote-monitoring mission is to issue its report on the Montenegro election later Monday.

Djukanovic, a former communist turned pro-Western supporter, has ruled the small Balkan state for 27 years with a firm hand either as its president or prime minister. He was pivotal in the country's split from much larger Serbia in the 2006 referendum.

"The police junta did everything to keep Djukanovic in power," opposition Democratic Front leader Andrija Mandic said, calling for a "transitional government that will not allow this brutal police interference in the electoral process."

Another Democratic Front leader, Nebojsa Medojevic, claimed that 2,500 opposition supporters were questioned by police on Sunday "just because they protested against Djukanovic."

He said the arrest of the 20 "alleged terrorists" on the election day was staged to rally Djukanovic's supporters and scare away the opponents.

"The oldest one is 73 years old and the youngest is 17, so you see what a farce this arrest is," Medojevic said.

The prosecutor's office said the group planned to attack people who gathered in front of the parliament when the vote results were proclaimed, then storm the building and declare the victory "of certain parties." The statement said they also planned to arrest Djukanovic.

News from © The Associated Press, 2016