Iran's nuclear chief Ali Akbar Salehi and his Russian counterpart attended the ceremony of loading uranium fuel into Iran's first nuclear power plant.
"The beginning of the first stage of the physical start-up has taken place," said Sergei Novikov, spokesman for Russia's state nuclear corporation Rosatom.
Russia has built and supplied the fuel for the $1-billion Bushehr plant, work on which was initially started by German company Siemens in the 1970s, before Iran's Islamic Revolution.
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.
The fuelling of Bushehr is a milestone in Iran's path to harness technology which it will reduce consumption of its abundant fossil fuels, allowing it to export more oil and gas and to prepare for the day when the minerals riches dry up.
The start of the 1,000-megawatt power reactor makes Iran the first country in the Middle East with a nuclear-energy facility, freeing more of its fossil fuels for export. Iran also becomes only the second Muslim state after Pakistan to have nuclear power, with ambitions to build enough plants to generate 20,000 megawatts within 20 years.