DENVER - An inmate in handcuffs and shackles jumped through a closed window in a third-floor courtroom, shattering the glass, plunging to the ground and seriously injuring himself, police said.

The man, in court on a misdemeanour assault charge, had been told to go before the judge at the Denver City and County Building, a witness said. "And he was just sitting there, turned around and he went headfirst out the third-storey window," Ty Berrien told KCNC-TV in Denver.

It wasn't immediately clear whether the man's goal was escape or suicide, police spokesman Sonny Jackson said.

Details on the inmate's injuries were not immediately released. He was lying on a grate outside the building when emergency workers arrived, said fire Lt. Phil Champagne.