The Department of Justice has been asked to investigate after prison guards allegedly forced inmates to engage in gladiator-style fights for "sadistic pleasure" and betting, in scenes the authorities said evoked Game of Thrones.

An inmate at the San Francisco County jail who weighed 150lb was allegedly forced to fight a 350lb inmate, under threat of being beaten up and shot with a Taser by guards, Jeff Adachi, the San Francisco County public defender, said in a press conference on Thursday.

Four sheriff's deputies have been accused by three inmates of orchestrating vicious fights for their own amusement while encouraging them with chants.