One of the most bitter battles over capital punishment in a generation has ended with victory for reformers after prosecutors in Pennsylvania abandoned their pursuit of the death penalty for the black radical Mumia Abu-Jamal.

Dubbed the "world's best known death row inmate", Abu-Jamal has had his death sentence commuted to life in prison with no chance of parole. The decision was announced on Wednesday just two days before the 30th anniversary of the murder of a white policeman for which he was convicted.

Lawyers representing Abu-Jamal said that the final admittance of defeat by the pro-death lobby after three decades of consistent effort to have him executed would send a message of hope to hundreds of other death row inmates across America. Abu-Jamal, who professes his innocence, has become one of the most vocal critics of the US practice of capital punishment through a series of books, including his 1995 work Live from Death Row.

Christina Swarns of the NAACP's legal defence fund, who has represented Abu-Jamal in recent appeals, said she hoped the outcome would be a harbinger for the decline of capital punishment throughout the country. "Here was a case where the prosecutors have fought so hard for so long to have Abu-Jamal executed, and the fact that they have failed will give hope to many others who do not have the benefit of his notoriety," she said.

Mumia Abu-Jamal (born Wesley Cook), aged 58, was sentenced to death for the 9 December 1981 murder of Daniel Faulkner. The police officer was shot several times at 4am in Philadelphia as he stopped a car belonging to Abu-Jamal's younger brother William Cook.

Abu-Jamal was working as a taxi driver at the time, and was parked across the street. He ran to his brother's assistance and was accused of shooting Faulkner in a blaze of gunfire in which Abu-Jamal was also wounded. A discharged hand pistol belonging to Abu-Jamal was found at the scene.

Over the past 30 years the condemned man and his supporters have questioned the conviction, claiming variously that the prisoner was framed by police, that another passenger in his brother's car was the shooter or that another unidentified man was responsible. The case had strong racial undertones, partly because the defendant was a former member of the Black Panthers and partly because of irregularities in the conduct of the trial.

Pennsylvania has executed three prisoners since the death penalty was re-introduced in 1976. The state senate is expected to debate a proposal to carry out a comprehensive study into the use of capital punishment locally, as a first step towards reviewing it.

There were accusations that the trial judge made a racist statement in court, and protests that the defendant was not allowed to represent himself.

In the ensuing years, human rights and other groups lined up to support Abu-Jamal in his battle to avoid execution, while police affiliations and the victim's family took the opposing position.

In the end, it was not a question of innocence or death that spared him. In 2001 a Pennsyulvania court vacated the death sentence on the grounds that the jury had been improperly briefed by the trial judge. Ten years of appeals followed, with the case bouncing around federal courts and ending up with the US supreme court, which in October refused to consider it further.

After so much legal wrangling, district attorney Seth Williams said he could no longer seek the death penalty, but remained unrepentant about his desire to do so. "There's never been any doubt in my mind that Mumia Abu-Jamal shot and killed officer Faulkner. I believe that the appropriate sentence was handed down by a jury of his peers... He will remain behind bars for the rest of his life, and that is where he belongs."

Maureen Faulkner, the police officer's widow, said the result merely underlined the justice system's "dirty little secret" – an inability to carry out executions. She accused the prisoner's supporters of being "dishonest cowards" and told the Associated Press that she was glad he would now be put in a general prison wing without protection.

"I am heartened that he will be taken from the protective cloister he has been living in all these years and begin living among his own kind – the thugs and common criminals that infest our prisons."