According to Alpha Research agency, Radev would have 63.9 per cent of the vote against 33.1 per cent for Anastas Gerdjikov, and 3 per cent preferred the option "none of the above."
Gallup International anticipated 65.7 per cent for Radev, 31.5 per cent for Gerdjikov, and 2.8 per cent "none of the above."
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The official results are expected to be announced on Wednesday, Xinhua news agency reported.
As many as 23 candidates participated in the first round of the presidential elections last Sunday. Radev, who was backed by several political formations such as Continue the Change coalition, Bulgarian Socialist Party and There Is Such A People, won with 49.42 per cent.
Gerdjikov, rector of Sofia University, who was supported by the center-right GERB party, ranked second with 22.83 per cent.
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Bulgaria's President is elected directly by the people for a period of five years. Radev's term will expire at the end of January 2022.