"Iran has joined the nuclear countries of the world," Mr. Ahmadinejad said during a large, carefully staged and nationally televised celebration in Mashhad, which included video presentations of each step of the nuclear process that he declared Iran had mastered. "The nuclear fuel cycle at the laboratory level has been completed, and uranium with the desired enrichment for nuclear power plants was achieved."
The White House, which has charged that Iran is secretly trying to develop fuel for nuclear weapons, at first reacted mildly to the announcement, saying Iran was "moving in the wrong direction." But later in the day it sounded a more ominous tone, with the National Security Council announcing that the United States would work with the United Nations Security Council "to deal with the significant threat posed by the regime's efforts to acquire nuclear weapons."
Outside experts said that while the country appears to have passed a milestone — one it has approached before with smaller-scale enrichment of uranium — the announcement may have had less to do with an engineering feat than with carefully timed political theater intended to convince the West that the program is unstoppable.
The declaration comes at a time of intense speculation in Washington that preliminary plans are advancing to take military action against Iran's nuclear sites if diplomacy fails, an idea Secretary of Defense Donald H. Rumsfeld dismissed Tuesday as "fantasy land."
The director general of the International Atomic Energy Agency, Mohamed ElBaradei, is scheduled to arrive in Tehran on Wednesday to make another appeal for Iran to halt its enrichment program and avoid a confrontation with the West. Iranian officials said Dr. ElBaradei would face a changed situation, and American officials said they suspected that Iran's strategy is to portray its effort as a fait accompli.
The news came as another major setback for the European nations that have pressed for three years to persuade Iran to halt its fuel production program, and for President Bush. On Monday, Mr. Bush repeated that his "stated goal" was that "we do not want the Iranians to have a nuclear weapon, the capacity to make a nuclear weapon, or the knowledge as to how to make a nuclear weapon."
For that reason, he has opposed allowing Iran to enrich uranium, even though Iran has signed the Nuclear Nonproliferation Treaty and has the right to produce fuel for nuclear power reactors.
If the Iranian declaration is correct, the enrichment and what appear to be rudimentary bomb-making documents that international inspectors have found in Iran suggest Iranians may now have most of the knowledge that Mr. Bush has sought to deny them.
At the least, they appear poised to be able eventually to expand enrichment on an industrial scale and, if they are determined to do so, enrich the uranium to levels necessary for an atomic weapon. But so far the quantities that the country has produced appear to be minuscule, and the enrichment level announced today — 3.5 percent — would work for producing power, not warheads.
International inspectors are stationed at Iran's main enrichment facility at Natanz, and presumably will be able to confirm or refute Iranian claims in coming days, assuming they have access to centrifuges.
Centrifuges are devices whose rotors spin very rapidly to enrich, or concentrate, a rare form of uranium known as uranium 235, which can then be used to fuel nuclear reactors or atom bombs. The 164 centrifuges Iran said it has strung together in a cascade are enough to test the technology, but with such a small number would take years to produce enough uranium for even one weapon.
"This 164 machines is more industrial," said a European diplomat who monitors Iran's program and spoke on the condition of anonymity. "But still, it's not like they haven't come close to achieving this in the past."
Despite claims on Tuesday of an enrichment breakthrough, Iran has in the past seven years repeatedly used centrifuges and lasers to enrich uranium, according to reports by the nuclear agency. But the amounts have apparently been small and the setups experimental.
Mr. Ahmadinejad reiterated that Iran's nuclear program was being developed for industrial and power purposes alone, and said his country "does not get its strength from nuclear arsenals."
But he did his best to turn the development to political advantage.
"I declare at this historic moment, with the blessings of God almighty and the efforts of our scientists, that we have mastered the nuclear fuel cycle on a laboratory scale and on Sunday our young scientists have produced enriched uranium required for nuclear plants," Mr. Ahmadinejad said.
"Access to the nuclear fuel cycle is a national demand and our people have repeatedly stressed that they want to have it."
His speech, given before a mural of doves in flight and a motto in English, was bracketed by recitations from the Koran and followed by chants of "God is Great."
Before he spoke, a small parade of men in traditional costumes danced as a thin silver box said to contain the first enriched uranium was carried to the stage. An announcer said the box would be preserved at a museum.
State-run television repeatedly showed footage of scientists in white uniforms working in what seemed to be a nuclear facility.
Mr. Ahmadinejad was careful to position Iran as operating within the existing proliferation rules, saying his country's nuclear activities have been "under complete, unprecedented" supervision by the atomic energy agency.
But he did not mention that he has restricted the access of those inspectors to some sites in recent months, and that inspectors have yet to receive explanations of the documents that appear to have bomb designs, or an explanation of the centrifuge equipment and designs the country bought in the mid-1990's from Abdul Qadeer Khan, the Pakistani nuclear engineer who helped start Iran's program.
Mr. Ahmadinejad said Iran would continue to allow inspectors to watch its progress. "Today we are interested to operate under I.A.E.A. supervision what has been achieved," he said. "And what is going to be achieved in the future is within the framework of the rights of the nation."
David Albright, president of the Institute for Science and International Security, a private research group in Washington, said that the announcement had been expected, but that the quantities of enriched uranium were probably small.
"They need to learn a lot more to produce it in significant quantities and they need to build a lot more centrifuges," he said.
Dr. ElBaradei is required to report back to the Security Council by April 28 on whether Iran has acceded to the demand late last month that it shut down its facilities within 30 days. One senior European diplomat said that in Iran, Dr. ElBaradei will "argue that they've gotten to enrichment, so it's time to shut the process down, and end the problem."
The question of whether Iran has the right to continue nuclear research has been at the heart of the conflict between Iran and the United States and its European allies this year. Iran rejected a Russian offer to enrich uranium in Russia for an Iranian nuclear plant because the deal would not allow any nuclear activity in Iran.