Iran “has opened a new chapter” in its ties with the world, President Hassan Rouhani said, hours after international nuclear sanctions were lifted.
The move came after the international nuclear watchdog, the IAEA, said Iran had complied with a deal designed to prevent it developing nuclear weapons.
Most Western governments hailed the move but Israel accused Tehran of still seeking to build a nuclear bomb.
Four dual US-Iran nationals were released from jail by Iran on Saturday.
They include Washington Post reporter Jason Rezaian, who was arrested in 2014 and jailed in November for espionage.
Early reports said all four had left the country.
However unnamed US officials later said that one of the four, Nosratollah Khosravi-Roodsari, was not on the plane headed for Switzerland.
Five key points of the nuclear deal
Will Tehran now get a McDonald’s?
Iran’s press ecstatic as sanctions end
Iran frees US citizens in prisoner exchange
Who is freed US reporter Jason Rezaian?
A fifth American, Matthew Trevithick, has also been released separately.
The US offered clemency to seven Iranians being held in the US for sanctions violation.