WASHINGTON (AFP) - A 57-year-old US prisoner who was sentenced to death for a murder he didn't commit walked free from court on Friday after spending 39 years behind bars, prosecutors said.

Ricky Jackson was only 18 in 1975, when he was convicted of murder in Cleveland, Ohio.

His prosecution was based largely on testimony of a 12-year-old witness who later admitted he had not seen the crime and had given a false statement.

A beaming Jackson left a court hearing a free man on Friday, even telling reporters he was prepared to forgive the witness whose evidence had put him behind bars.

"I guess a lot of people will want me to hate that person and carry animosity towards them but I don't," Jackson said, in footage shown on the Cleveland Plain Dealer newspaper website.

"People see him as a grown man today, but in 1975 he was a 12-year-old kid and he was manipulated and coerced by the police and they used him to get us in prison.

"As far as that young man is concerned, I wish him the best. I don't hate him, I just wish he has a good life."

According to the National Registry of Exonerations, a University of Michigan project which tracks wrongful convictions, Jackson's 39 years in prison make him the longest serving exoneree in history.

"It's extraordinary. I'm glad to be out... It has been an emotional roller coaster. The English language doesn't have the words to express how I'm feeling right now," Jackson said.

He had been convicted with two other men of shooting dead a businessman outside a store in Cleveland in 1974. His death sentence had been commuted to life in prison in 1977.

One of Jackson's co-accused was released in 2003. The other remained in custody but will now be freed after prosecutors in Cuyahoga County dropped all charges against the men.

Joe Frolik, a spokesman for the prosecutors office told AFP "all charges were dropped and he was free to go. The decision was taken after the recanting of the witness who was 12."