He told the Saham news website that because of a previous problem, his father would have stay in an emergency care unit for an additional 48 hours. The elder Mr. Karroubi has been in and out of the hospital over the past year with heart problems.

Image Mehdi Karroubi, one of Iran’s most prominent opposition leaders, in 2009. He ended a one-day hunger strike on Thursday but remained under house arrest, his son said. Credit... Vahid Salemi/Associated Press

Mr. Karroubi’s success in extracting the concessions from the Iranian establishment is small but significant. Analysts said they still expected it would be quite a while before a public trial started — “perhaps too long for Mr. Karroubi, who is old and sick,” said Farshad Ghorbanpour, a political analyst close to the government.

The government’s concessions were a political lift for President Hassan Rouhani, a moderate whose supporters have long demanded the release of Mr. Karroubi; another opposition leader, Mir Hossein Mousavi; and Mr. Mousavi’s wife, Zahra Rahnavard. Mr. Mousavi and Ms. Rahnavard have also been held since 2011.

It is widely thought that Iran’s supreme leader, Ayatollah Ali Khamenei, opposes their release and objects to a trial. Neither of the two men has been formally charged, nor has Ms. Rahnavard.

Mr. Karroubi, a cleric and a former speaker of Parliament, and Mr. Mousavi, a former prime minister, both ran for president in the disputed 2009 election. After the incumbent, Mahmoud Ahmadinejad, declared victory, huge street protests rocked Tehran and other cities in what became known as the Green Revolution. Mr. Karroubi and Mr. Mousavi emerged as its unofficial leaders.