Florida Department of Corrections via Reuters John Errol Ferguson, 65, shown in this undated photograph.

MIAMI - A Florida man who has spent 35 years on death row for killing eight people was executed on Monday despite a last-minute appeal by lawyers claiming he was insane.

John Errol Ferguson, 65, who was convicted of first-degree murder and sentenced to death in 1978 for a pair of killing sprees, was pronounced dead at 6:17 p.m. EDT from lethal injection, said Misty Cash, a spokeswoman for the Florida Department of Corrections.

Hours before his execution, the U.S. Supreme Court denied Ferguson a stay of execution. The National Alliance on Mental Illness (NAMI) filed an amicus brief last week, along with three Florida mental health organizations, asking the top court to halt the execution, arguing Ferguson had a long history of severe mental illness.

The brief argued Ferguson's execution would violate the Eighth Amendment to the U.S. Constitution requiring an individual to have a rational understanding of why he is being put to death and the effect of the death penalty.

"Mr. Ferguson is insane and incompetent for execution by any measure," his attorney, Christopher Handman, said in a statement after the court's decision on Monday.

"He has a fixed delusion that he is the 'Prince of God' who cannot be killed and will rise up after his execution to fight alongside Jesus and save America from a communist plot," Handman said. He has no rational understanding of the reason for his execution or the effect the death penalty will have upon him."

In his last statement, Ferguson uttered, "I just want everyone to know, I am the 'Prince of God' and I will rise again," Cash said.

In July 1977, Ferguson fatally shot six people execution-style during a drug-related home robbery in a northern Miami suburb. Six months later, he killed two teenagers after they left a church meeting.

State psychiatrists and other medical professionals have diagnosed Ferguson as a paranoid schizophrenic with a long history of mental illness, according to his defense team.

Courts, however, have repeatedly rejected claims he was too mentally ill to be executed.

Florida Governor Rick Scott signed Ferguson's death warrant in September, but a few weeks later delayed the execution while a team of physicians met to decide whether Ferguson was mentally competent.

After a 90-minute examination and brief consultation a panel of psychiatrists determined that Ferguson was sane. A state circuit judge agreed in a ruling.

The U.S. 11th Circuit Court of Appeals in May rejected his appeal, ruling that Ferguson was mentally competent.

"That most people would characterize Ferguson's Prince-of-God belief, in the vernacular, as 'crazy' does not mean that someone who holds that belief is not competent to be executed," the appeals court found.