Iran on Sunday hanged a man said to be affiliated to an exiled opposition group, state media reported, despite international pressure on the Islamic republic to halt the execution.
According to the official IRNA news agency, Gholamreza Khosravi Savadjani was convicted of "waging war against God" (moharebeh) by helping the People's Mujahedeen Organisation of Iran (PMOI).
The announcement of the hanging came just hours after Amnesty International said Khosravi Savadjani's trial in 2010 had been unfair.
The rights group said the condemned man's family were informed by prison officials on Saturday that they must go to a jail west of Tehran, sparking fears his execution may be imminent.
Khosravi Savadjani was until then being held in solitary confinement at Evin Prison in the capital. Death row prisoners in Iran are generally transferred to isolation units before their executions take place.
Prior to his death, Amnesty said the execution would be a breach of domestic and international law, as Khosravi Savadjani -- held since 2008 -- should have benefited from a subsequent law that imposed lighter penalties for the crimes he was convicted of.
The PMOI was founded in the 1960s to oppose the pro-western shah.  
After the 1979 Islamic revolution that ousted the shah, the PMOI took up arms against Iran's clerical rulers and Tehran holds it responsible for murdering thousands of Iranian civilians and officials.
Iran says PMOI members currently in exile in Iraq should be extradited to face charges.
Khosravi Savadjani was arrested in 1981 and jailed for several years. He was detained again in 2008 for having contact with the PMOI and has been in custody since.
According to the Iranian judiciary, documents, including photos and papers from sensitive facilities such as military bases, were recovered when Khosravi Savadjani was arrested. These had been given to the PMOI and their affiliated media, officials said.
Khosravi Savadjani had also been accused of facilitating financial aid for the opposition group. He was convicted by a revolutionary court and the verdict was later upheld by a branch of Iran's Supreme Court.
Amnesty International said Khosravi Savadjani had reportedly been held for more than 40 months in solitary confinement in various detention centres.
"Yet again Iranian authorities are about to execute a man who did not even receive a fair trial," Hassiba Hadj Sahraoui, Amnesty's deputy director for the Middle East and North Africa, said on Saturday.
Iran remains the second biggest executioner in the world, after China, according to the United Nations.