Iran's leader warned that attack on the reactor would be met with a global and "painful" response, but reaction to the news in Washington was muted.

“We recognize that the Bushehr reactor is designed to provide civilian nuclear power and do not view it as a proliferation risk,” State Department spokesman Darby Holladay told Agence France-Presse on Saturday.

Iranian news outlets, though, touted the development as the Islamic Republic exercising its nuclear freedom and a new era of nuclear cooperation with Russia, which has pledged to safeguard the plant against producing weapons-grade materials. Iran has long claimed its nuclear program is for peaceful purposes, which has been met by suspicion and sanctions in much of the world. Still, Iran has defiantly pressed forward on what it claims is its right to nuclear development.

Russia had signed a $1 billion contract to build the Bushehr plant, which overlooks the Persian Gulf, back in 1995. Iran has begun enriching uranium to the 3.5 percent level need to fuel the plant.

"Transfer of nuclear fuel to the main building of the Bushehr nuclear power plant annulled all enemies' resolutions, sanction and threats," deputy head of the parliament's National Security and Foreign Policy Commission Hossein Ebrahimi told the semi-official Fars News Agency on Saturday.

Iran's Foreign Ministry spokesman Ramin Mehman-Parast told the official Islamic Republic News Agency that the next nuclear project would move forward at high speed with "comprehensiveness."

"Launching the project after overcoming serious obstacles created by several countries helralds the fact that Iran will accelerate the implementation of its upcoming programs," IRNA reported.

Iranian Defense Minister Brigadier General Ahmad Vahidi said Tehran would unveil a defense project of "great importance" Sunday, Mehr News Agency reported. "Now, more than ever, we are vigorously standing up to arrogant regimes," Vahidi said.

Israel's response to the news contrasted with the benign response from the State Department.

"It is totally unacceptable that a country that blatantly violates decisions of the United Nations Security Council and the International Atomic Energy Agency, and ignores its commitment to the Non-Proliferation Treaty charter, will enjoy the fruits of using nuclear energy," Foreign Ministry spokesman Yossi Levy said, according to The Jerusalem Post.

"The international community," he added, "must increase pressure on Iran, so that it will obey international decisions, halt its activity in the field of enrichment and construction of heavy water reactors, and will fully reply to the accusations raised against it."

Iranian President Mahmoud Ahmadinejad told Qatari newspaper Al-Sharq at the plant's opening that any action against the facilities would bring a "harsh and painful" response.

"Our possibilities would be limitless and would encompass the whole world," said Ahmadinejad.

Of Israel, the Iranian president added, "I don't believe their American masters would let them attack."