Mr Djukanovic was prime minister during a tense October 2016 parliamentary election when authorities said they thwarted a pro-Russian coup attempt created to prevent the country from joining North Atlantic Treaty Organisation. His party officials announced late on Sunday that Djukanovic, who has dominated politics in Montenegro for almost 25 years, had secured victory. President Filip Vujanovic, also of the ruling party, was not running due to term limits.
Polls suggest the 56-year-old could win more than half of the votes on Sunday and avoid a run-off.
The 56-year-old economist led Montenegro to independence from Serbia in 2016 and into North Atlantic Treaty Organisation last year - now he wants to take the predominantly Orthodox country, a part of which has strong pro-Russia sympathies, into the European Union. "Looking forward to years of cooperation in ensuring this future for our two peoples", Thaci wrote.
The opposition accuses Djukanovic of being linked to the mafia, which he denies.
Mladen Bojanic, the second ranked candidate supported by larger part of opposition parties, won 32 percent, while Draginja Vuksanovic from the Social-Democratic Party of Montenegro won 8 percent.
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"I understand this election result as a confirmation of Montenegro's evolution to achieve European values, European standards and full membership in the European Union", Djukanovic said before opening a bottle of champagne.
On Sunday Bojanic said he voted to "put an end to the reign of an autocrat who wants to turn Montenegro into a dictatorship".
Djukanovic has claimed the opposition want to turn the country into a "Russian province" and threaten Montenegro's multicultural way of life. If both win equal number of votes in the runoff election, a third round will be appointed in seven days.
For Djukanovic, however, the choice between Brussels and Moscow is crucial to Montenegro's development.
Turnout at the presidential elections, according to the electoral commission, was 64 percent, meaning that 327,865 people voted.