Iran's new ultra-conservative President Mahmoud Ahmadinejad has been confirmed in office, following his surprise election victory in July.
Mr Ahmadinejad is a former Revolutionary Guard
His endorsement came as Tehran said it hoped to restart its controversial nuclear activities on Wednesday.
One of his first challenges will be a row with the US and EU, which want an end to Iran's nuclear programme.
The former mayor of Tehran succeeds reformist Mohammad Khatami, who leaves office after eight years in power.
Following his endorsement by Iran's supreme leader Ayatollah Ali Khamenei, 49-year-old President Ahmadinejad said he will lobby for the elimination of weapons of mass destruction around the world and for international justice.
With Mr Ahmadinejad in power it means an end to the reform period in Iran - hardliners now control all the institutions of power, says the BBC's Frances Harrison in Tehran.
But our correspondent says he inherits what looks set to be a major foreign policy crisis, triggered by Iran's decision this week to resume conversion of uranium.
A spokesman for Iran's Supreme National Security Council said a request by the UN's nuclear watchdog to wait a week to allow surveillance systems to be installed was unacceptable.
Ali Aghamohammadi said Iranian experts and UN inspectors were at work at the plant near Isfahan in central Iran.
"They are doing the executive work and we hope that today we will be able to restart the activities," he told reporters.
"One week is not acceptable for Iran for the installation of equipment."
Tehran suspended all uranium conversion and enrichment activities in November, but has always insisted the suspension was temporary.
Enriched uranium can be used to fuel nuclear power stations, but can also provide material for nuclear weapons.
The move prompted warnings from the international community urging Iran to rethink and warning of serious consequences, including referral to the UN Security Council.
Iran says it has a right to a peaceful nuclear programme. The US believes it is seeking nuclear weapons.
EU negotiators are due to submit shortly full proposals for nuclear, economic and political co-operation with Iran, provided it ends all nuclear activities.
On Saturday, Mr Ahmadinejad will take the oath of office in parliament.
Mr Ahmadinejad was a little-known figure on the international stage before the election, our correspondent says.
The incoming president denies being a hostage-taker
Few thought he was a frontrunner, he says, but he won in a run-off by a huge margin of 10m votes.
Mr Ahmadinejad is a former Revolutionary Guard and was most recently the mayor of Tehran.
But since winning the elections, Mr Ahmadinejad has been plagued by stories that he has a shady past, adds our correspondent.
He was accused of taking US diplomats hostage after the Islamic revolution as a student, and faced allegations of being involved in the killing of a Kurdish opposition leader.
He has denied the accusations and his supporters say this is part of a smear campaign against the president-elect orchestrated by the West.