Protesters gathered outside the Council of Ministers building in Sofia, demanding that president Iliana Yotova and prime minister Rumen Radev resign.
Sofia municipality told BTA that the protest had not been registered. Police have stepped up their presence in the area.
The crowd chanted "Resign", "Traitors" and "Prison".
They say Yotova and Radev are failing to deal with ships hit and sunk by drones in Bulgaria's exclusive economic zone in the Black Sea. The protesters are convinced that all of Europe knows the strikes were Russian, and that only Bulgaria's president and government pretend not to know.