By Irina Ivanova

SOFIA (Reuters) - Bulgaria's ruling GERB party candidate, Rosen Plevneliev, won a presidential run-off against his Socialist opponent, preliminary official results showed on Monday, strengthening his party's grip on power before a parliamentary poll.

Second-placed Ivailo Kalfin, former foreign minister in the previous Socialist-led cabinet, won 47.4 percent after 99.9 percent of the votes cast in Sunday's election were counted, data from the central electoral commission showed.

The victory will boost Prime Minister Boiko Borisov before the election in 2013 in the poorest European Union country, which is recovering from a prolonged recession that hit incomes and raised unemployment.

The narrow margin between the newly-elected president and Kalfin, however, signals that the Socialists are rebounding from the defeat they suffered in 2009 and will most likely be GERB's major rival in the vote.

The campaign was marred by rallies against the Roma minority and corruption, highlighting GERB's struggle to tackle graft and to kickstart the economy.

The Organization for Security and Cooperation in Europe (OSCE), which monitored the election, reiterated concerns about suspected vote-buying and pressure on voters.

"It is ... important to address shortcomings such as persistent allegations of vote-buying," mission head Vadim Zhdanovich said in a statement.

"This should be properly investigated and long-standing recommendations should also be considered."

Before the vote, Plevneliev was construction minister, and he became more popular than Borisov thanks to the speedy building of highways.

He has promised to fight graft in the public sector by moving services online and to work toward approving a bill authorizing the confiscation of illegally obtained assets.

"I became president of Bulgaria because in the last two years I worked for society and the people to see a new kind of politics," Plevneliev told state broadcaster BNT, vowing to produce "concrete results."

The 47-year-old engineer will replace outgoing president Georgi Parvanov, a Socialist, denying the opposition a high-profile, though largely ceremonial, position and the chance to veto bills or appointments initiated by the cabinet.

Plevneliev's victory is also likely to strengthen Bulgaria's EU and NATO integration.

Although he has said that Sofia will not pursue an anti-Russian policy, Plevneliev he has fallen short of openly supporting major Russian-backed energy projects like the South Stream gas pipeline or the planned 2,000 megawatt nuclear power plant Belene.