Cleric Hassan Rouhani has officially replaced Mahmoud Ahmadinejad as president of Iran.
Mr Rouhani's election was endorsed by Supreme Leader Ayatollah Ali Khamenei at a ceremony at a mosque in the capital, Tehran, on Saturday.
He won the presidential poll in June, promising to reform and to put an end to Iran's international isolation.
The ceremony marked the handover of power, but his public inauguration does not take place until Sunday.
Mr Rouhani is the seventh president of the republic. AFP reports he faces grave challenges abroad and at home over the economy and isolation resulting from the policies of his predecessor.
The BBC reports many Iranians believe Mr Ahmadinejad put Iran on the path to economic ruin and confrontation with the outside world.
While he may be taking over as president, a BBC correspondent says correspondent he will not be Iran's main decision-maker: the Supreme Leader, not the president, has the final say in Iran.
Ob Friday, Mr Rouhani, 64, said Israeli occupation was an "old wound on the body of the Islamic world", as Iran marked its annual Jerusalem Day.
Iran has denied Israel's right to exist since the 1979 Islamic revolution.