Iran can now produce nuclear fuel on an industrial scale, President Ahmadinejad has announced, in a move likely to further strain tensions with the West
The Natanz plant is thought to have room for 50,000 centrifuges
He gave no details of Iran's capacity, but some officials said 3,000 uranium gas enrichment centrifuges were running at the Natanz plant in central Iran.
Mr Ahmadinejad's speech came as Iran celebrated nuclear technology day.
Iran maintains its nuclear programme is purely peaceful, but the West fears it wants to build atomic bombs.
The UN has passed two packages of sanctions against Iran for refusing to suspend its uranium enrichment programme.
A US spokesman said the White House was "very concerned" about the Iranian announcement.
"Iran continues to defy the international community and further isolate itself by expanding its nuclear programme, rather than suspending uranium enrichment," said Gordon Johndroe, spokesman for the White House National Security Council.
UN Secretary General Ban Ki-moon urged Iran to follow the Security Council resolution.
"I sincerely hope that, even at this time, when the Iranian government is undergoing Security Council sanctions, that they should engage in dialogue.... It is very important for any member country to fully comply with the Security Council resolution," he told reporters.
An EU spokesman renewed calls for Iran to suspend uranium enrichment, AFP news agency reported.
"This does nothing to change our position - Iran must co-operate fully with the IAEA and follow the United Nations resolutions," said Ton Van Lierop, a spokesman for the European Commission.
"With great honour, I declare that as of today our dear country has joined the nuclear club of nations and can produce nuclear fuel on an industrial scale," Mr Ahmadinejad told the audience at Natanz.
He did not say how many centrifuges - the machines that spin uranium gas in order to enrich it to levels needed for fuel - were now operational at Natanz.
Ahmadinejad did not disclose any figures on Iran's progress
Iran announced in February that it had set up two cascades of 164 centrifuges each at Natanz. It said it planned to have 3,000 centrifuges by the end of last month.
Ali Larijani, Iran's chief nuclear negotiator with the West, said at Natanz on Monday that Iran had begun injecting gas into many of the centrifuges, without specifying the number.
Some other officials said 3,000 centrifuges had been installed, the BBC's Frances Harrison at Natanz reports.
The most sensitive areas at Natanz, deep underground, are thought to be halls that can hold up to 50,000 centrifuges.
Mr Larijani also warned that Iran would have no choice but to review its membership of the Nuclear Non-Proliferation Treaty if further pressure was applied by the West over its nuclear programme.
Journalists and diplomats were invited to the event at Natanz, but EU diplomats boycotted them in protest at Iran's refusal to comply with UN demands to end its uranium enrichment programme.
A year ago, Iran announced that it had produced enriched uranium for the first time. It said it had enriched only to 3.5% - the level needed for nuclear fuel but way below that needed to make a nuclear bomb.