Robert "Mudman" Simon, 48, was the first to meet such an end since the moratorium on capital punishment in the United States was lifted 23 years ago.

Ambrose Harris, 47, sentenced to death for raping and killing a 22-year-old artist, attacked Simon, who shot dead a police officer four years ago, said investigators. They clashed in the New Jersey state prison's recreation area where inmates can play chess and select books from a library.

"Harris apparently beat Simon down with his fists then kicked him until he was unconscious," said an investigator. "Then he climbed on a table and stomped on his head." The victim, suspect and three witnesses were all convicted murderers.

Simon, a member of the Warlocks bikers' gang, was the first to die on death row since New Jersey readopted capital punishment in 1982. No executions have been carried out because of legal appeals and a tradition of liberalism in the state's supreme court.

Prison officers were reported to have taken at least five minutes to intervene in the fight. But John Cunningham, vice-president of the corrections officers' union, said guards could not react immediately because the rules prevented them from entering the caged recreation area without riot gear.

"Prison is a violent place," he said. "The department did everything it could to prevent this from happening."

Harris had a history of attacking guards and prisoners.

Simon was jailed in 1982 for 10 to 20 years after killing a girlfriend who refused to have sex with gang members. He killed another inmate in prison but was acquitted on the grounds of self-defence. Weeks after being freed on parole, he killed the policeman.

"I think Robert Simon would rather have died on the floor fighting than take his lethal injection," said John Call, his former lawyer. "That was his lifestyle."