Campaigners have called for an international day of protest on Saturday to demand the release of researcher Ahmadreza Djalali, who could be executed this month after being convicted of spying in Iran.
Mr Djalali, 51, a Swedish-Iranian citizen, was arrested in 2016 and convicted of sending confidential details to Israeli intelligence service Mossad, allegations his supporters say are without merit.
His detention follows a pattern of hostage-taking by the Iranian regime who have released western and dual-citizen inmates held on trumped-up charges in return for payments and in prisoner swap deals.
The Campaign to Free Political Prisoners in Iran called for protests on Saturday to put pressure on Tehran.
“On this day, we call on everyone to support Ahmadreza Djalali and his family by holding a protest rally anywhere around the world, and demand his unconditional release,” said the group, which was set up by former political prisoners.
It also called on the Swedish and other western governments to step up their efforts to halt Iran's planned execution of the disaster medicine researcher.
A statement from the campaign group said the Iranian regime "must come under international pressure and be isolated to stop the execution of Ahmadreza Djalali, and release him from prison”.
Mr Djalali’s lawyer has said he could be executed at any time. Iran’s semi-official news agency ISNA said that the death sentence would be carried out by May 21.
The threat of imminent execution coincided with the end of a trial in Sweden of Hamid Nouri, a former Iranian regime official who is accused of involvement in state-backed killing of thousands of opposition activists in Iranian prisons in 1988. A verdict is expected in July.
Speculation has circulated that Iran is seeking a swap of the two prisoners, although Iran has publicly declared it is not seeking a deal.
Swedish Foreign Minister Ann Linde spoke to her Iranian counterpart Hossein Amirabdollahian last week, shortly after initial reports of Mr Djalali’s death sentence.
Mr Djalali was convicted after making confessions obtained under torture, the United Nations says. The organisation says he should be released and paid compensation.
Pictures of a gaunt Mr Djalali in prison highlighted how he has suffered with a series of health problems during his six years in prison, during which he has spent long periods in solitary confinement.