The UN is expected to confirm on Saturday Iran's compliance with last July's landmark nuclear deal, allowing the lifting of sanctions and what Iran's foreign minister called a "good day for the world."
"Today is a good day for the Iranian people as sanctions will be lifted today and also a good day for the region," Mohammad Javad Zarif said as he arrived in Vienna ahead of the anticipated announcement by the UN atomic watchdog.
The International Atomic Energy Agency report is expected to confirm that Iran has dramatically scaled down its nuclear programme as agreed in the hard-won July 14 deal agreed in Vienna.
This, combined with ultra-close IAEA inspections, extend to at least a year -- from a few months previously -- how long Iran would need to make one nuclear bomb's worth of fissile material.
Iran has always denied wanting nuclear weapons, saying its activities are exclusively for peaceful purposes such as power generation.
The IAEA's green light means that a raft of US, EU and UN sanctions on the Islamic republic can be lifted, allowing oil exports to resume and opening up the 80-million-strong country to business.
Zarif and EU foreign policy chief Federica Mogherini were due to declare after the IAEA's announcement that the agreement has now entered into force.
US Secretary of State John Kerry was also expected in the Austrian capital, diplomats said, as well as top representatives from the other parties to the deal Russia, China, Britain, France and Germany.
The Vienna agreement was sealed after two years of rollercoaster negotiations following the June 2013 election of Iranian President Hassan Rouhani.
The highly complex deal drew a line under a standoff dating back to 2002 marked by failed diplomatic initiatives, ever-tighter sanctions, defiant nuclear expansion by Iran and threats of military action.
In addition it put Iran and the United States on the road to better relations some 35 years after the Islamic revolution that toppled the US-backed shah, and at a particularly explosive time in the Middle East.
Zarif said Saturday that the accord had removed from the Middle East "the shadow of a baseless confrontation", according to the ISNA news agency.
The agreement, heralded as US President Barack Obama's biggest major foreign policy triumph, has by no means been universally cheered, however.
Obama's Republican opponents charge that it fails to do enough to ensure Iran will never get the bomb, a complaint shared by Israel, Iran's arch foe widely assumed to have nuclear weapons itself.
Sunni Saudi Arabia, Iran's other great regional rival, is also alarmed at the prospect of warmer US-Iran ties and of predominantly Shiite Iran, newly flush with oil revenues, increasing its influence.
Kerry sought to allay his Saudi counterpart's fears on Thursday in London, telling Adel al-Jubeir that the two nations' "friendship... remains a lynchpin of our efforts in the region".
Already Saudi Arabia and Iran, fighting a proxy war in Yemen and key players in the Syrian conflict, are at daggers drawn following Saudi Arabia's execution of a Shiite cleric in early January and the subsequent ransacking of the Saudi embassy in Tehran.
Iran's imminent return to the oil market has also contributed to the sharp slide in the price of crude to 12-year lows of under $30 per barrel this week, putting Saudi Arabia's public finances under strain.
The lifting of sanctions on Iran is "going to put 500,000 barrels per day more on the market", said James Williams of WTRG Economics. Some analysts believe $20 oil is on the horizon.
The deal has more than a decade to run, which is likely to be a bumpy road, experts say, not least if more hardline governments take power in Tehran or Washington.
The two countries are still far from being best friends, as witnessed by Iran's recent capture of 10 US sailors in Iranian waters, although their improved relations did help ensure their swift release.
Iran violated a UN resolution in October when it test-launched a medium-range missile capable of carrying a nuclear warhead, a UN panel of experts concluded in a report in December.
A "snapback" mechanism ensures that many of the sanctions can be swiftly reimposed, and a special joint commission is meant to handle any misunderstandings.
"Iran may test the boundaries of the agreement. It is critical that violations do not go unpunished, or the deal could be killed by a thousand paper cuts," said Kelsey Davenport of the Arms Control Association.
"At the same time, punitive actions for violations on both sides should be proportionate, and differentiated from technical missteps, which may occur under such a complex agreement," she told AFP.