MIAMI (AFP) - Eminem savaged Mr Donald Trump as a racist and warned fans not to support the US President as the rap superstar appeared at the BET Hip Hop Awards on Tuesday.

The top-selling rapper of all time made a rare and unexpected appearance in a recorded video aired during hip-hop's premier awards show.

With a hoodie over his head and anger in his voice, he rapped freestyle from inside a parking ramp as an entourage stood guard behind him.

Eminem in his profanity-laden salvo decried Mr Trump as both incompetent and prejudiced.

He pledged solidarity with former San Francisco 49ers quarterback Colin Kaepernick, whose protests against racism during the US national anthem have triggered a row as Mr Trump demands that athletes stand for the flag.

Eminem, whose music is rarely overtly political, made the intervention a year after releasing his last song - also a denunciation of Mr Trump recorded in the run-up to the election.