A former leader of the Black Panthers has been released from prison more than four decades after being convicted of killing a Baltimore police officer.

Sixty-seven-year-old Marshall "Eddie" Conway was freed from the prison in Jessup on Tuesday. It came after state prosecutors agreed to change his life sentence to time served and probation.

Conway is one of dozens of inmates who have been released after Maryland's Court of Appeals ruled in 2012 that judges had given improper instructions to juries before 1980.

Conway was convicted in 1971 of fatally shooting 35-year-old Donald Sager as Sager and another officer sat in a marked patrol car.

He has always denied any participation in the attack.