Bulgarians vote in a closely fought presidential election on Sunday that could plunge the Black Sea state into political instability and push it toward closer ties with Russia.

Opinion polls show ruling party candidate Tsetska Tsacheva, 58, is likely to narrowly win the ballot but lose a subsequent runoff to Rumen Radev, 53, a Socialist Party ally who wants to end European Union sanctions against Moscow.

"The vote on Sunday will show whether we have the right to rule the country in the next two years," Prime Minister Boiko Borisov told supporters at the close of the campaign.

If Tsacheva does lose, Radev's Socialists and other opposition groupings could try to unseat the center-right GERB party's minority government and trigger early parliamentary elections, observers say.

That would lead to months of political uncertainty and a further slowdown in reforms, much needed after the collapse of Bulgaria's fourth largest bank and massive anti-corruption protests that helped GERB win parliamentary elections in 2014.

Public disappointment over reform progress eroded support for GERB, while Tsacheva is seen as lacking the charisma of Prime Minister Borisov.

"I will vote for General Radev because he hasn't been involved in dirty political games, so far," said 68-year-old pensioner Dimitar Tashev. "He is not a member of any political party and he deserves a chance."

Voting starts at 07.000 a.m. local time (0500 GMT) and polls close at 20.00 p.m.

Latest opinion polls showed parliament speaker Tsacheva winning with 27.2-26.3 percent of the vote, against Radev's 22.5-23.1 percent.

Nineteen other candidates are also running and most polls show Radev could win a runoff vote on Nov. 13, which has to be held between top two contenders if no one wins an overall majority.

Radev, a former air force commander, has campaigned largely on a promise of closer ties with Moscow, arguing the small European Union state - fully dependent on Russian energy - should hedge its bets when it comes to strategic alliances.

Bulgaria, seen as the Kremlin's most pliable satellite in Soviet times, has long been an anomaly in Europe, a country within EU and NATO structures yet feeling close to Russia.

"Bulgaria is a small country, we do not have resources and it is naive to go against Russia," said Kalin Dragiev, 49, shop owner. "Politicians must accept that Bulgaria badly needs Russia ...I will probably vote for Radev but I still think that Tsacheva is the favorite."

EU sanctions imposed on Moscow over its role in pro-Russian separatist conflict in Ukraine have hurt NATO member Bulgaria financially.

Most power in the country of 7.2 million rests with the prime minister and parliament, but the president leads the armed forces and can veto legislation, sign international treaties and appoint ambassadors.