A woman casts her ballot at the polling station in Montenegro's capital Podgorica, Sunday, April 15, 2018. Voters in Montenegro are casting ballots in a presidential election, with former Prime Minister Milo Djukanovic expected to win after his party defied Russia and took the small Balkan nation into NATO last year. (AP Photo/Risto Bozovic)

PODGORICA, Montenegro - Montenegro's ruling party leader Milo Djukanovic swept a presidential election on Sunday, preliminary results showed, and he pledged to keep the small Balkan country firmly on a European path after it joined NATO last year in defiance of Russia.

Djukanovic won 54 per cent of the ballots, securing a victory in the first round and avoiding a runoff, according to results released by the independent Center for Monitoring and Research. His main opponent, Mladen Bojanic, won 33 per cent.

If confirmed in the official vote count, the result will present a major boost for Djukanovic and his ruling Democratic Party of Socialists.

Sunday's vote, the first since Montenegro joined the Western military alliance in December, was seen as a test for Djukanovic, who favours European integration over closer ties to traditional ally Moscow.

"We have accomplished an important victory for (Montenegro's) European future," Djukanovic told cheering supporters, adding he saw the triumph "as the confirmation of Montenegro's strong determination to continue on the European road."

Djukanovic's party earlier declared him the winner as supporters took to the streets to celebrate. Crowds drove in cars around the capital, Podgorica, honking horns and waving flags as fireworks lit the sky.

"Milo Djukanovic is the new president of Montenegro," said Milos Nikolic, of the DPS. "This is a great victory, a historic victory."

Challenger Bojanic, who was backed by several opposition groups, including pro-Russian ones, vowed to continue his struggle against Djukanovic, describing him as "the man holding Montenegro and its institutions hostage."

"I will continue to fight to free Montenegro of Djukanovic and his dictatorship," Bojanic said. "I am appealing to opposition voters not to view this as a defeat but as a basis for further struggle."

Djukanovic, the country's dominant politician, and his party have ruled Montenegro for nearly 30 years. President Filip Vujanovic, also of the ruling party, was not running due to term limits.

About 530,000 voters were choosing among several candidates in the Adriatic Sea nation that used to be part of Yugoslavia. For the first time in the staunchly conservative nation, a female candidate also ran for the presidency, winning 8 per cent of the vote.

Djukanovic has served both as prime minister and president in several mandates since becoming the youngest head of government in Europe at the age of 29 in 1991.

He was prime minister during a tense October 2016 parliamentary election when authorities said they thwarted a pro-Russian coup attempt designed to prevent the country from joining NATO.

Djukanovic led Montenegro to independence from much-larger Serbia in 2006 and was behind the NATO bid, which Moscow strongly opposed.

He promised Sunday to work to overcome divisions within Montenegro, a predominantly Orthodox Christian Slavic nation where many still cherish historic links to Russia.

Bojanic, an economic expert and former lawmaker, has accused the ruling party of corruption and links to organized crime following a spike in crime-related violence.

The fractured opposition parties supporting Bojanic include the pro-Russian Democratic Front, whose two main leaders are on trial for taking part in the alleged 2016 coup attempt.

Two Russian citizens also are being tried in absentia for the plot, which prosecutors said included a plan to assassinate Djukanovic. The Kremlin has denied involvement.

News from © The Associated Press, 2018