By Tsvetelia Tsolova and Angel Krasimirov
SOFIA (Reuters) - Socialist-backed candidate Rumen Radev, who has called for an end to European Union sanctions against Russia, has won the first round of Bulgaria's presidential election, partial official results showed on Monday.
Radev's close-fought victory over ruling party candidate Tsetska Tsacheva makes the former air force commander favorite to win a run-off on Sunday, a result that could push the Black Sea NATO member state politically closer to Russia.
Results from 95 percent of polling stations showed Radev, 53, winning 25.7 percent of the vote. Center-right candidate Tsacheva, who had been expected to win narrowly, won 22 percent.
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 signaled he may quit if his candidate Tsacheva loses.
Bulgaria's president is a largely ceremonial figure, but can also influence policy, veto legislation and sign international treaties.
Radev, a jet fighter pilot and novice to politics, has tapped into public anger with political elites and fears about immigration, vowing not to let the Balkan country become a "migrant ghetto". Some migrants have entered Bulgaria from neighboring Turkey en route for wealthier western Europe.
"The hope for a change is already tangible. People want to see more security and well-functioning institutions," Radev said on Monday.
Radev has made clear he believes Bulgaria, which joined NATO in 2004 and the EU in 2007, should focus more on reviving economic and political ties with its old ally Russia, which has been under Western sanctions since its annexation of Crimea from Ukraine in 2014.
"In their frustration, my rivals are turning into fear-mongers," Radev said. "For them, the Euro-Atlantic mantra also means (being an) enemy of Russia. For me, this is an unhealthy position."
The Bulgarian vote comes amid growing divisions within the EU over whether to maintain sanctions against Russia or even to expand them over Moscow's role in the Syria conflict. Greece, Cyprus and Hungary are among Moscow's closest EU allies.
The Bulgarian Socialist party (BSP), successor to the once-mighty Communists and the country's main opposition party, is keen to repair damaged ties with Moscow and has accused the outgoing center-right president, Rosen Plevneliev, of dividing Bulgarians with his criticism of Russian policies.
The Kremlin's most loyal satellite in the Cold War era, Bulgaria, a country of 7.2 million, remains today a popular holiday destination for Russians, some of whom have bought property there, especially on the Black Sea coast.
Bulgaria is almost entirely dependent on Russia for its energy supplies and many Bulgarians feel a strong cultural affinity for the country, with which they share the Cyrillic script and Orthodox Christianity.
Tsacheva, who if elected would be Bulgaria's first female head of state, secured the support on Monday of the right-wing Reformist Bloc, the junior partner in Borisov's coalition which had fielded its own candidate in the first round.
She repeated her concern that victory for Radev could put Bulgaria at odds with its EU and NATO allies.
"There are two options - to allow Bulgaria slide back into its dark past of ideological lies and submission to foreign interests or ... to make sure that Bulgaria stays where it belongs, among free European countries," Tsacheva said.
"We cannot allow a red general to lead Bulgaria," said Bozhidar Lukarski, leader of the Union of Democratic Forces, one of the five parties making up the Reformist Bloc.
Radev and Tsacheva both agreed to take part in a public debate before the run-off.
Borisov's minority coalition government took office two years ago, bringing political stability after a year of anti-corruption protests and prompting the EU to resume aid flows it had suspended.
But his failure to show tangible results in fighting rampant graft and overhauling inefficient sectors like healthcare and education have eroded his popularity.
(Reporting by Tsvetelia Tsolova and Angel Krasimirov; Editing by Mark Trevelyan and Gareth Jones)