Iliana Iotova and Kiril Valchev lead the presidential runoff with 52.5% of the vote, Alpha Research says. The figures are as of 7pm.
The ticket of Andrey Gyurov and Georgi Kandev is second with 43.4%.
The agency says 4.1% of those who voted chose the "I don't support anyone" option on the ballot.
Turnout at the same time was 29.1%.