A German rights group urged a comprehensive investigation into the death of a man who was beaten up by the police in the city of Mannheim.

"A 47-year-old man died during a brutal police check in #Mannheim on Monday," the Amadeu Antonio Foundation said on Twitter.

"Allegations are growing that there could be a racist motive here. The case leaves us stunned - and must be fully investigated," the group said.

Authorities have confirmed that the man was a German citizen of Croatian origin, who was a psychiatric patient at Mannheim's Central Institute of Mental Health.

The exact cause of his death remained unclear, as he also had other health issues, including cardiac insufficiency, according to the investigators.

A forensic autopsy was conducted to determine the cause of death.

Videos circulating on Twitter appeared to show police officers using excessive force to restrain the victim, and one officer punching him in the face several times, while he was lying on the ground.