A monument to the Soviet Army has been daubed in pink paint on the anniversary of the invasion of Czechoslovakia during the Prague Spring.
An inscription beneath reads: "Bulgaria Apologises".
Near the beginning of the Cold War, eight communist states formed the Warsaw Pact in response to the integration of West Germany into NATO.
On 21 August 1968, armies of five of these countries – the Soviet Union, Poland, Hungary, Bulgaria and East Germany - invaded Czechoslovakia to crush democratic reforms underway in the state.
Seventy-two people were killed and hundreds were injured, many severely.
A number of non-violent protests to the invasion took place across the Warsaw Pact countries - including the case of a student. Jan Palach, who set himself on fire.
The resistance caused Soviet Union's plan to oust first minister, Alexander Dubček who was responsible for the reforms, to falter and he was allowed to remain in office (he would be sacked the next year however).
Bulgaria was the last of the invading countries to apologise in 1990.