Iran has begun fuelling its first nuclear power plant after decades of delay and amid international fears it is seeking to make an atomic bomb and not just electricity.
Russia has built and supplied the fuel for the Bushehr plant, work on which was initially started by German company Siemens in the 1970s, before Iran's Islamic Revolution.
Iranian television showed live pictures of Iran's nuclear chief Ali Akbar Salehi and his Russian counterpart watching a fuel rod assembly being prepared for insertion into the reactor.
'Despite all the pressures, sanctions and hardships imposed by Western nations, we are now witnessing the start-up of the largest symbol of Iran's peaceful nuclear activities,' Mr Salehi told a news conference.
The US criticised Moscow earlier this year for pushing ahead with the start-up plans amid persistent Iranian defiance over its nuclear activities.
Moscow supported a fourth UN Security Council resolution in June, which imposed new sanctions and called for Iran to stop its uranium enrichment which, some countries fear could lead it to obtain nuclear weapons.
But Russia says a deal with Iran, under which it will both supply Bushehr and take back the spent fuel - which could be used to make weapons-grade plutonium - means it cannot help any Iranian efforts to build a bomb.