(Reuters) - Georgia executed a convicted killer on Tuesday despite the objections of his lawyers who argued he was mentally disabled and should have been spared the death penalty, the state's top lawyer said.

Warren Lee Hill, 54, was pronounced dead by lethal injection at 7:55 p.m. (0055 GMT Wednesday), after the U.S. Supreme Court denied a request to put his execution on hold, Georgia Attorney General Sam Olens said in a statement.

The Georgia Board of Pardons and Paroles refused on Tuesday to stop the execution, as did the state Supreme Court last week.

Hill was condemned for beating fellow inmate Joseph Handspike to death in August 1990. At the time, Hill was serving a life sentence for the 1985 shooting death of his 18-year-old girlfriend, Myra Wright.

Five thousand people signed a petition seeking to spare his life and it was presented to the state parole board, his lawyers said.

Hill's supporters included former President Jimmy Carter and his wife, Rosalynn, the American Bar Association, the American Civil Liberties Union, the European Union and the Georgia chapter of the NAACP, according to his attorneys.

"This execution is an abomination," Hill's attorney, Brian Kammer, said in a statement. "The memory of Mr. Hill's illegal execution will live on as a moral stain on the people of this State and on the courts that allowed this to happen."

Georgia prosecutors said Hill had the capacity to understand his crime, noting he served in the U.S. Navy and was a father figure to his younger siblings.

In 1988, Georgia was the first state to ban the execution of mentally disabled inmates, but critics say its standard for proving disability is too strict.

Experts generally define mental disability as having a score of 70 or below on intelligence tests. Hill scored 69 on one intelligence test and in the 70s on others, according to court records.

He had the mental capacity of an 11-year-old, his attorney said.

Hill's lawyers filed affidavits from three doctors who found Hill competent 13 years ago but now believed he was mentally disabled.

His attorneys argued for more time to appeal in light of a U.S. Supreme Court decision last year that found Florida's IQ test standard for assessing mental disability was too rigid.

Hill was the fifth U.S. inmate executed this year and the second in Georgia, according to the Death Penalty Information Center.