The Associated Press by a self-identified hacker group called The Justice of Ali, a guard looks at surveillance screens taken over by the group, at Evin prison in Tehran, Iran. AP
Olivier Vandecasteele, 41, is suffering "monstruous injustice" at the hands of Iranian authorities, who arrested him in Tehran in February, they said in a statement.
He is one of several Western nationals detained in Iran, in what foreign-based activists say is a bid to extract concessions.
Iran is seeking the return of one of its diplomats, Assadollah Assadi, who is in prison in Belgium serving a 20-year sentence after being found guilty last year of masterminding a foiled 2018 bomb plot outside Paris.
Vandecasteele has been kept in solitary confinement since being seized and is suffering a growing number of physical afflictions, his family said.
They included "major weight loss, haematomas on his toes, fingernail loss, and worrying dental and gastric problems," they said.
He started his hunger strike two weeks ago, consuming nothing more than bread and water once a day.
His last communication with his family was on September 1, and they said they feared he was suffering "irreversible harm" from the "disgraceful" detention conditions.
The bomb plot Assadi was convicted for was an attempted attack on a conference outside Paris held by an exiled Iranian opposition group.
Iran insists that Assadi should enjoy diplomatic immunity, even though he was arrested in Germany, away from Austria where he was accredited to work in Iran's embassy in Vienna.
Belgium this year agreed a prisoner-swap treaty with Iran seen as opening the door for Assadi to be sent home if Tehran releases Vandecasteele.
But the treaty has generated controversy, with some Belgian politicians, Iranian opposition groups and the United States all coming out against it as allowing an Iranian "terrorist" to escape justice.