Iranian hard-liners using arrests to wrest back power from moderate President Rouhani, analysts say.
Four Iranian-Americans, including Washington Post reporter Jason Rezaian, have been freed in Iran in a prisoner swap deal, Iranian and US officials said.
Rezaian, who was taken into custody in July 2014 and convicted on espionage charges last year, were expected to be flown to Switzerland along with former US Marine Amir Hekmati, pastor Saeed Abedini and Nosratollah Khosravi-Roodsari, US officials said.
Washington Post and a US official confirmed on Sunday that the plane carrying those four had taken off from Tehran.
In return for the Americans, the US will pardon or drop charges against seven Iranians – six of whom are dual US-Iranian citizens – accused or convicted of violating US sanctions.
In addition, the US will drop Interpol “red notices” – essentially arrest warrants – on 14 Iranian fugitives it has sought, officials said.
US student Matthew Trevithick was released in Iran independently of the exchange on Saturday and has already left the country.
The swap came as diplomats gathered in Vienna to announce the lifting of international sanctions and bring the country of 80 million people back to the global economic stage.
The International Atomic Energy Agency announced in Vienna that Tehran had complied with the requirements of a deal reached last year, under which it was to curb its nuclear programme in return for the lifting of sanctions imposed by the United States, United Nations and European Union.