Iran has begun loading fuel into its first nuclear power station in a ceremony attended by Russian officials.
Russia will operate the Bushehr plant in southern Iran, supplying its nuclear fuel and taking away the nuclear waste.
Iran has been subject to four rounds of UN sanctions because of its separate, uranium enrichment programme.
Experts say that as long as the plant is Russian-operated, there is little immediate threat of its fuel being diverted to make bombs.
The West fears Tehran wants to build a nuclear weapon, but Iran insists its plans are for peaceful energy production.
The Bushehr facility has taken 35 years to build and has been plagued by delays.
"Despite all pressure, sanctions and hardships imposed by western nations, we are now witnessing the startup of the largest symbol of Iran's peaceful nuclear activities," Iranian nuclear chief Ali Akbar Salehi told journalists at the plant.
The BBC's Jon Leyne says Iranian officials have promoted Saturday's launch as a victory for the Islamic republic against its enemies. Nationwide celebrations are planned to mark the event.
But Professor Ali Ansari, an Iran expert at the University of St Andrews, in Scotland, said Tehran was likely to exaggerate the importance of the start-up at Bushehr.
"It will obviously have a very theatrical opening but the delays have meant that the power plant is a very old model and the contribution to the national grid is very small," he said.
The Bushehr plant should begin producing electricity in about a month, experts say.
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