According to the country's constitution, a candidate should receive 51 percent of the votes to become president.
The voter turnout in the country's fifth presidential elections stood at 45.3 percent, the Central Election Commission said earlier.
The first round was won by the Bulgarian Socialist Party candidate, former Bulgarian Air Force Commander Maj. Gen. Rumen Radev, who received 24.8 percent of the votes.
Parliament Speaker Tsetska Tsacheva backed by the ruling GERB party won 23.5 percent votes.
The second round of the elections will be held on Nov. 13.
Prime Minister Boyko Borisov had announced that his party would resign if Tsacheva did not win the elections.