Iran says it is suspending its uranium enrichment programme, in line with a deadline agreed with European nations.
Iran denies claims that it wants to build nuclear weapons
The suspension has been welcomed as "a good step in the right direction" by the head of the UN's nuclear watchdog.
Earlier, the head of Iran's nuclear energy body said work would stop at two nuclear plants in Isfahan and Natanz.
Tehran agreed a week ago to suspend its enrichment operations in a deal with the three European nations to allay fears about its nuclear ambitions.
Iranian state television announced on Monday that work on uranium enrichment had been halted.
The International Atomic Energy Agency (IAEA) is now checking that "everything has been stopped", said Mohamed ElBaradei, director general of the UN-backed agency.
The IAEA is due to discuss Iran's compliance at a meeting on 25 November.
Iran still risks a referral to the UN Security council - which could lead to sanctions - if it fails to comply, the British foreign secretary warned on Monday.
Britain, France and Germany brokered the deal with Tehran to suspend uranium enrichment.
"If there is a failure by Iran to meet its obligations then
Britain and also Germany and France reserve our collective right to refer the matter to the Security Council," said UK Foreign Secretary Jack Straw.
Iran has reacted angrily to recent reports it was speeding up uranium enrichment before the suspension took effect.
Tehran also hit back at outgoing US Secretary of State Colin Powell's assertion that it was trying to adapt its ballistic missiles to carry nuclear warheads.
"I believe Powell has understood his remarks were false," Iran's nuclear chief Hassan Rohani told state television on Sunday. "Such claims are totally baseless."
But Mr Powell refused to back down, telling reporters on a flight to the Middle East: "I stick with it."
Iran has always denied US claims that it is developing a nuclear weapons programme, saying its intentions are peaceful.
The head of Iran's nuclear agency, Gholam Reza Aghazadeh, said enrichment activities would stop as agreed.
"I believe Iran's Atomic Energy Organisation has carried out whatever measures are required for confidence-building," he told reporters.
He also rejected diplomats' claims that Iran was exploiting the window until Monday to rush production at Isfahan processing facility.
"The Isfahan plant has a specified capacity and cannot
operate beyond what has been planned," he said.
"The plant has no enrichment activity. Raw materials are
just processed there and it has started activities in this
field since a few months ago."
"They need to build confidence and the suspension of uranium enrichment is a good step in the right direction," IAEA head Mohamed ElBaradei told BBC Radio on Monday.
He said that Iran had made two tons of uranium gas used in enrichment, but that this was not enough to produce a nuclear weapon.
Washington has been at the forefront of moves to persuade the IAEA to refer the country to the UN Security Council, which has the power to impose sanctions.