PODGORICA, Montenegro — Montenegrin officials blocked popular messaging services WhatsApp and Viber during the country’s parliamentary election, a ban that drew allegations of interference from opposition politicians and concern from European election watchers Monday.

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

Authorities said they blocked Viber and WhatsApp for several hours during Sunday’s inconclusive election because “unlawful marketing” was being spread through the networks.

Prime Minister Milo Djukanovic’s long-ruling party won the most votes in the contest, 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 current course toward the West or turns back to traditional ally Russia.

The tense election was marked by the arrest of 20 people, including a former commander of Serbia’s special police forces, suspected of planning politically motivated armed attacks against Djukanovic and his supporters. Opposition leaders claim thousands of their supporters were rounded up by the police on election day.

While pointing out the need for further improvements, the OSCE vote-monitoring mission said in its report Monday the elections “were held in a competitive environment and fundamental freedoms were generally respected.”

But the report also expressed concern over the blocking of Viber and WhatsApp. European parliamentarian Marietje Schaake said she was worried that such a draconian measure had apparently been deployed.

Schaake said in an email that such moves “must not be used to silence opposition or to limit the freedom of assembly or speech. It is crucial that the EU looks into the details of what took place, and ensures accountability.”

Messages sent to WhatsApp and its owner Facebook Inc. weren’t immediately returned. Viber, owned by Japanese tech company Rakuten Inc., didn’t immediately provide comment on the reported outage.

Djukanovic, a former communist turned pro-West 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.

Opposition 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” was staged to rally Djukanovic’s supporters and scare away 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.

State TV said Monday the group was handed a 72-hour pre-investigation detention.

AP Cybersecurity Writer Raphael Satter contributed to this report from Paris.