Iranian and Russian engineers have begun loading fuel into Iran's first nuclear power plant.
This is a major milestone as Tehran forges ahead with its atomic programme despite United Nations sanctions.
The week-long operation will load fuel into the reactor at the Bushehr power plant in southern Iran.
It ends years of foot-dragging by Russia, which signed a £640 million contract in 1995 to build the plant but delayed its completion several times.
There is international suspicion that Iran is seeking an atomic bomb and not just electricity.
Iran remains under intense international pressure to stop uranium enrichment, something the West says it no longer needs to do as it can acquire nuclear fuel from abroad.
State television showed live pictures of the country's nuclear chief Ali Akbar Salehi and his Russian counterpart looking on at what appeared to be a fuel rod suspended from the ceiling.
Iranian officials say it will take two to three months before the plant starts producing electricity once the uranium-packed fuel rods are moved into the reactor.
Tehran's refusal to cease enrichment has resulted in a series of UN sanctions and tougher unilateral measures by the United States, the European Union and elsewhere.
Iran insists it is not seeking a bomb and says it has a sovereign right to nuclear technology and uranium enrichment.