Release delayed for man held 43 years in solitary confinement
Albert Woodfox was held in solitary confinement for 43 years on charges of murdering a prison guard (AFP)

A US prosecutor has blocked the release of a former Black Panther who spent a record 43 years in solitary confinement in a case that has provoked outrage among rights groups.

Louisiana's top prosecutor on Tuesday blocked the release of 68-year-old black prisoner Albert Woodfox, whose conditions of detention -- confined 23 hours a day -- drew outrage from rights activists.

A federal judge had ordered his unconditional release in a strongly-worded ruling Monday that barred any further trial on charges of murdering a prison guard, for which Woodfox received two convictions, both later overturned.

But Louisiana's attorney general filed an appeal and won a temporary stay on Tuesday, blocking his release until at least Friday.

Woodfox's 43 years in solitary confinement are the longest of any US prisoner despite his lengthy court battles to prove his innocence.

Rights activists say the long imprisonment under difficult conditions of Woodfox and two former fellow inmates -- also former Black Panther activists -- exposed failings in the country's justice system.

The three were convicted of the murder of a guard during a 1972 prison riot.

They became known as the "Angola Three" as their years in solitary confinement at the notorious Louisiana State Penitentiary at Angola drew attention to their avowals of innocence.

Woodfox is the last of the group to remain behind bars.

Amnesty International, which has supported the members for years, welcomed the judge's decision to release him as a "momentous step toward justice" to right a legal process "riddled with flaws", in a statement from senior campaigner.

The Angola Three fought what they said were cruel prison conditions at the Louisiana facility built on a former slave plantation.

- Strong words -

One of the trio, Herman Wallace, died in 2013 due to cancer, a few months after being freed from four decades in solitary confinement.

Robert King, the third member, was freed in 2001.

The group said the solitary confinement caused serious mental and physical trauma.

In November, the US Court of Appeals for the Fifth Circuit overturned Woodfox's second conviction, but he was charged again at state level in February.

Woodfox has long maintained his innocence, citing a witness who said he was not involved in the murder, as well as a scientific review of evidence at the scene and a polygraph test that found he had truthfully denied his involvement.

"Evidence supporting claims of Mr Woodfox's innocence gives this Court even more reason to question his two previously overturned convictions," US District Judge James Brady wrote in ordering his release.

Brady listed five "exceptional circumstances" for Woodfox to be released unconditionally and immediately, rather than the usual conditional release pending trial.

In addition to Woodfox's age and "poor health," the judge also cited his "lack of confidence in the state to provide a fair third trial".

He also cited the prejudice done to Woodfox in spending over 40 years in solitary confinement, and finally the fact that he has already been tried twice and would otherwise face his third trial for a crime that occurred over 40 years ago.