Supreme Leader Ayatollah Ali Khamenei has formally endorsed Mahmoud Ahmadinejad as winner of Iran's presidential election in June, according to reports on state television.
The ayatollah backed the incumbent two days before the official inauguration amid tales of post-election torture.
Leading opposition figures and two former presidents were not at the ceremony, state-run al-Alam TV said.
The BBC reports Mr Ahmadinejad now faces a struggle to assemble a team of ministers acceptable to parliament.
Since the violent aftermath of the disputed election on 12 June there has been open disagreement among senior figures in the Iranian political establishment.
In June, Ayatollah Khamenei offered his backing to Mr Ahmadinejad at the height of the post-election protests.
But senior and respected figures, including the two former presidents, Mohammad Khatami and Akbar Hashemi Rafsanjani, have spoken out against the process.
Neither man was at Monday's endorsement ceremony, al-Alam TV said, while opposition figurehead Mir-Hossein Mousavi and another defeated presidential candidate, Mehdi Karroubi, also stayed away.