The South African rapper Costa Titch died on stage while performing, police said Sunday, as they opened an investigation into the circumstances of the 28-year-old's sudden death.

The artist "collapsed while he was performing" on Saturday evening at the Ultra South Africa concert in the Johannesburg suburb of Nasrec, police told AFP.

They said a post-mortem would establish the cause of death.

Costa Titch scored a major hit with "Big Flexa," which has more than 45 million Youtube views, showcasing the amapiano local subgenre of house music blending house, jazz and lounge music.

Videos on social media of his concert Saturday show him performing with his microphone in hand when he appears to fall. He continues singing but collapses again, prompting other artists to come to his aid.

Costa Titch, whose real name is Costa Tsobanoglou, died a month on from the assassination of another popular South African rapper Kiernan Forbes, known as AKA.

Forbes was shot dead outside a Durban restaurant and an investigation is ongoing into what has been seen as a likely contract killing.

Tributes swiftly appeared for Titch on Sunday with Julius Malema, leader of radical leftist party EFF, posting an image of a broken heart alongside Costa Titch's name on social media.

The Southern African Music Rights Organisation wrote on Twitter: "SAMRO is saddened by the passing of popular rapper Costa Tsobanoglou, better known as Costa Titch. Heartfelt condolences to his family, friends and broader music industry."

"RIP, Costa Titch. Great talent gone too soon," tweeted rapper Da L.E.S.