Pro-Russia, anti-migration candidate Rumen Radev won 25.8 percent of the vote in first round of the election.
Socialist-backed opposition candidate Rumen Radev, who has called for an end to European Union sanctions against Russia, won the first round of Bulgaria's presidential election.
Radev's close-fought victory over ruling party candidate Tsetska Tsacheva on Monday makes the former air force commander favourite to win a run-off on Sunday, a result that could bring the Black Sea state politically closer to Russia.
Radev, a jet fighter pilot and novice to politics, has tapped into public anger with political elites and fears about immigration, and vowed not to make the Balkan country a "migrant ghetto".
"The hope for a change is already tangible. People want to see more security and well functioning institutions," he said after his win.
The country of 7.2 million people already has warmer ties with Russia than most of its EU peers. Radev has made it clear he believes NATO member Bulgaria should focus more on its economic and political ties with Moscow, which has been under EU sanctions since it annexed Crimea from Ukraine in 2014.
Results from 86 percent of polling stations showed Radev, 53, winning 25.8 percent of the vote. Centre-right candidate Tsacheva, who had been expected to win narrowly, was on 22 percent, according to the national election commission.
A Radev victory in the run-off could usher in months of political instability, including a possible snap parliamentary ballot, after Prime Minister Boiko Borisov signalled that he may quit if his candidate, Tsacheva, loses.
Tsacheva appealed for support in the run-off, saying victory for her opponent could put Bulgaria at odds with its EU and NATO allies and trigger a spike in corruption.
"It is important to know what kind of Bulgaria we want to see - a country isolated internationally, with EU aid frozen and ... corruption - things that the Socialists trigger when they take power - or a Bulgaria whose voice is heard in Brussels," she said.