LAGOS, Nigeria — Human rights lawyers have filed an urgent appeal to try to prevent Nigerian authorities from executing a convicted armed robber by firing squad, two organizations said Thursday.

The suit filed Wednesday argues it is unconstitutional to deny an appeal to the man sentenced by a military tribunal in 1995, when Nigeria was under a military dictatorship notorious for its unfair trials and torturing confessions from detainees.

The lawyers said Thankgod Ebhos, 53, has been in prison since 1988, nearly half his life. In 1995, he was tried under a section of the Robbery and Firearms Act that does not allow for an appeal against the judgment of a military tribunal.

On Monday, Ebhos was dragged to the gallows where he watched four fellow death-row inmates hanged at Benin State Prison in southern Edo state. The hangings ended a seven-year moratorium on executions in Nigeria, where the death penalty is mandatory for the crimes of armed robbery and murder.

"The planned execution by firing squad ... after witnessing the execution of four inmates constitutes mental torture and amounts to inhuman and degrading treatment" that contravenes Nigeria's constitution, the appeal says.

Ebhos was reprieved because prison officials were not equipped to shoot him, Edo state Attorney General Henry Idahagbon said.

"Why they have been in prison so long and not executed I do not know and cannot say," he told The Associated Press in a telephone interview. He blamed "some people not wanting to do their job" — an apparent reference to President Goodluck Jonathan's recent statement urging governors to sign death warrants, no matter how painful they find it.

The first federal official to address the subject, Justice Minister Mohammed Adoke, blamed state governors, saying, "Most of the issues involving death sentences are state offenses. We cannot operate a federation and impose on the various states how to run their laws."

He told a press conference that his ministry would continue to encourage states to abolish the death penalty "so at the appropriate time when we have the key stakeholders buy into it, the law will be reviewed and this will be a thing of the past."

Idahagbon, who is also the state commissioner for justice, said he did not know when Ebhos may be executed. "I'm speculating but suspect prison authorities have to work with the military authorities to have the convict shot," he said.

The statement announcing the appeal, which was signed by director Justine Ijeomah of the Human Rights, Social Development and Environmental Foundation and attorney Chino Obiagwu of the Legal Defense and Assistance Project, said the original death warrant signed by the military administrator of northern Kaduna state stipulates that Ebhos should be executed in public in Kaduna city by the 1st Mechanized Division of the Nigerian Army.

"Any execution of the sentence of death passed on him by the military tribunal will be unlawful and further undermine respect for the rule of law in Nigeria," the statement said.

It called on the president to commute Ebhos' sentence to a term of imprisonment.

The appeal was filed at the Federal High Court in Abuja, Nigeria's capital.