Unknown artists have painted a Soviet Army monument in the capital, Sofia, pink in honour of the anniversary of the Prague spring.

Residents discovered on Wednesday morning that the figures of Soviet soldiers had been brightly coloured and an inscription added below in Bulgarian and Czech that read: "Bulgaria apologises."

On 21 August 1968, armies of five Warsaw Pact countries – the Soviet Union, Poland, Hungary, Bulgaria and East Germany – invaded Czechoslovakia to crush democratic reforms known as the Prague spring. One hundred and eight people were killed with 500 seriously injured.

The same monument in 2011. Photograph: Oleg Popov/AP

Bulgaria, an ally of the Soviet Union for decades, was the first country to call for the invasion and the last one to formally apologise for its participation, in 1990.

In 2011, grafitti artists transformed the soldiers in the monument into popular superheroes and cartoon characters, including Superman, Santa Claus and Ronald McDonald.