Nationwide celebrations are planned for the fuel loading at the Bushehr facility in southern Iran, while Russia pledges to safeguard the plant and prevent spent nuclear fuel from being shifted to a possible weapons program.

"The startup operations will be a big success for Iran," lawmaker Javad Karimi said in Tehran. "It also shows Iran's resolve and capability in pursuing its nuclear activities."

The West has not sought to block the reactor startup as part of its confrontations over Iran's nuclear agenda, a clash that has resulted in repeated rounds of U.N. sanctions against Tehran. Washington and other nations do not specifically object to Tehran's ability to build peaceful reactors that are under international scrutiny.

However, hard-liners see it as defiance of U.N. Security Council sanctions that seek to slow Iran's nuclear advances.

What concerns America and others - including Russia - is Iran's refusal to halt uranium enrichment, a process that can be used to make fuel for nuclear arms.

AP