"This is one of the final stages in the physical launch of the reactor," Vladislav Bochkov, a spokesman for the Russian company Atomstroyexport, said.
The generating unit at the Bushehr reactor was brought up to the "minimum controllable level of power" Sunday, the BBC reported.
The reactor has been at the center of controversy as Israel and other nations have voiced concerns the reactor could by used by Iran as part of an effort to develop nuclear weapons.
A member of an Iranian parliamentary commission overseeing Bushehr said Monday "final tests" were being conducted.
On Tuesday, Atomstroyexport said it had initiated "a self-supporting chain reaction" in the "active zone" of the plant's first reactor.
The facility would start providing power to Iran's national grid within two months, the country's Fars news agency reported.