Bulgaria's second-largest opposition party DPS has said it throws its support behind the socialist-backed candidate in the presidential elections.

Rumen Radev has emerged as a winner of the first round, defeating Prime Minister Boyko Borisov's candidate Tsetska Tsacheva.

The latter was initially thought to count on support from the Reformist Bloc (RB) coalition, whose own candidate Traycho Traykov is estimated to have earned some 6% of the vote. The decision was announced by the RB's co-chair, Bozhidar Lukarski, but Traykov subsequently distanced himself from his comments.

Krasimir Karakachanov, who got the third-best result in the first round, said on Sunday evening the nationalist alliance that nominated him would decide whom to support over the course of the week.

Veselin Mareshki, the businessman who surprisingly made it to the fourth place in the presidential race, said at a press conference he could personally vote for GERB's candidate. 

While the presidential vote is a majority election, the two candidates will be seeking to attract support from the electorate of big parties.

A number of polling agencies predicted before the first round, however, that the runoff's outcome will be in the hands of the DPS, its electorate being known for discipline in voting.