International sanctions on Iran have been lifted after a watchdog confirmed it had complied with a deal designed to prevent it developing nuclear weapons.
The EU foreign policy chief, Federica Mogherini, said the deal would contribute to improved regional and international peace and security.
The landmark deal between Iran and world powers was agreed last July.
Lifting the sanctions will unfreeze billions of dollars of assets and allow Iran's oil to be sold internationally.
The international nuclear watchdog, the IAEA, said its inspectors had verified that Tehran had taken the required steps.
As part of the deal, Iran had to drastically reduce its number of centrifuges and dismantle a heavy-water reactor near the town of Arak, both of which could be used in creating nuclear weapons.
Iran has always maintained its nuclear programme is peaceful, but opponents of the deal - such as some US Republicans - say it does not do enough to ensure the country cannot develop a nuclear bomb.
The US Secretary of State John Kerry has ordered that US nuclear-related economic sanctions against Iran be lifted.
Speaking in Vienna where he had been holding talks with his Iranian counterpart, Mr Kerry said Iran had "undertaken significant steps" which many people "doubted would ever come to pass".
"I thank God for this blessing and bow to the greatness of the patient nation of Iran" - Iranian President Hassan Rouhani on Twitter
"Today, as a result of the actions taken since last July, the United States, our friends and allies in the Middle East, in the entire world are safer because the threat of a nuclear weapon has been reduced" - US Secretary of State John Kerry
"Even after signing the nuclear deal, Iran has not relinquished its ambition to obtain nuclear weapons, and continues to act to destabilise the Middle East and spread terror throughout the world while violating its international commitments" - Israeli Prime Minister Benjamin Netanyahu
"Today, the Obama administration will begin lifting economic sanctions on the world's leading state sponsor of terrorism" - US Republican House Speaker Paul Ryan in a statement
"Years of patient and persistent diplomacy, and difficult technical work, have borne fruit as we now implement the deal" - British Foreign Secretary Philip Hammond
Earlier on Saturday it emerged that Iran had released Washington Post reporter Jason Rezaian and three other Iranian-American prisoners in an apparent prisoner swap with the United States.
Rezaian, 39, was jailed on charges, including espionage, last November.
The US said it was offering clemency to seven Iranians being held in the US for sanctions violation.
Mr Kerry said said he was "very happy to say that as we speak five Americans have been released from custody and they should be on their way home to their families shortly".
A fifth American, Matthew Trevithick, was also released on Saturday.
President Barack Obama would give more details of the releases later, Mr Kerry said.
The IAEA said it had installed a device at the Natanz plant to monitor Iran's uranium enrichment activities in real time, in order to verify that uranium enrichment levels were kept at up to 3.67% as agreed in the deal with world powers.
The organisation's head, Yukiya Amano, is to visit Tehran on Sunday for meetings with President Rouhani and other officials.
What is the nuclear deal?
In July 2015, Iran agreed a landmark nuclear deal with six world powers to limit its sensitive nuclear activities for more than a decade in return for the lifting of crippling sanctions. The US is confident the agreement will prevent Iran from obtaining a nuclear weapon. Iran says it has the right to nuclear energy - and stresses that its nuclear programme is for peaceful purposes only.
What does Iran stand to gain?
The sanctions have cost Iran more than more than $160bn (£102bn) in oil revenue since 2012 alone. Once they are lifted, the country will be able to resume selling oil on international markets and using the global financial system for trade. Iran has the fourth largest oil reserves in the world and the energy industry is braced for lower prices. Iran will also be able to access more than $100bn in assets frozen overseas.