A new exhibition titled '75 years since the Liberation of Eastern Europe from Nazism' opened to the general public at the Russian Culture and Information Centre in Sofia on Monday.
The Russian Ambassador Anatoly Makarov gave a speech praising the "friendship" between the two countries.
Protesters picketed the Embassy-organised event, calling Russians 'occupiers' and holding up various signs bearing anti-Russian slogans.
Last Tuesday, the Bulgarian Foreign Ministry criticised the exhibition as a "dubious historical claim". While acknowledging the role RUssian played in defeating Nazi Germany in WWII, they argued that the country lived under "half a century of repression" from the Soviets.
An uprising on 9 September 1944, a day after the Red Army marched into Bulgaria, brought the country into the Soviet orbit.
Since the collapse of communism, the Balkan country became a member state in the EU and the NATO military alliance.