Nigeria has hanged at least four convicted criminals in the West African nation's first known executions since 2006.
Human rights group LEPAD told The Associated Press that inmates at Benin City Prison were traumatized by the sound of screams and thuds.
In a telephone interview, a lawyer for the group, Chino Obiagwu, said that a fifth man was due to be hanged but received a brief reprieve when the executioner had technical problems with the gallows.
However, an email from someone in the prison suggested that he too may eventually have been hanged: "The sound of the gallows is heard, we believe execution has been done, is over."
A statement on the Amnesty International website appeared to confirm the report:
Amnesty International has now received credible reports that authorities in the state of Edo in southern Nigerian have hanged four men in Benin City Prison on Monday.
The justice commissioner in Edo state in southern Nigeria told the official News Agency of Nigeria that the four prisoners were all convicted of either armed robbery or murder.
The commissioner, Henry Idahagbon, said the men's appeals had been exhausted and their death warrants were signed.
"The court gave the judgment this afternoon that the execution could go ahead and the prison authority went ahead."
Amnesty International's deputy Africa director, Lucy Freeman, said the hangings marked "a truly dark day for human rights" in Africa's most populous nation.