The US state of Texas has executed its 500th convict since it resumed carrying out capital punishment in 1982.

Kimberly McCarthy, 52, was put to death on Wednesday evening for the murder of her 71-year-old retired college professor Dorothy Booth in 1997.

McCarthy was declared dead by lethal injection at 6:37pm (23:37) GMT in the Walls Unit prison in the small town of Huntsville, prison officials said.

"As the lethal drugs began to take effect her last words were 'God is good,'" the Texas Department of Criminal Justice said in a press release.

McCarthy stabbed Booth with a butcher's knife and beat her with a candelabra before, police said, she cut off Booth's finger to take her wedding ring.

Around 30 death penalty opponents gathered outside the prison during the execution.

More than 1,300 people have been executed across US since the Supreme Court lifted a moratorium on the application of the death penalty in 1976. More than a third of the executions were carried out in Texas.

'Racial discrimination'

McCarthy, who is black, received two last-minute reprieves in January and April due to allegations of racial discrimination during the selection of what became her all-white jury.

But after a Texas appeals court refused to reopen the case, she ran out of both options and time.

"If there was something to appeal, I would," her attorney Maurie Levin told the AFP news agency.

"For procedural reasons, the claims were never reviewed on the merits" she said.

Donna Aldred, Booth's daughter, told the reporters that her mother "was an incredible person who was taken before her time."

With increased debate in recent years over wrongful convictions, some states have halted executions entirely. However, 32 states have the death penalty on the books.