The Iranian president said the plant, which sharpened international concern over Tehran's nuclear ambitions, was 18 months from starting operations and that Western powers would regret accusing Iran of hiding it.
"We don't have any problems with (IAEA) inspections of the facility. We have no fears," he said at a news conference in New York, where he was attending the UN General Assembly.
US President Barack Obama, British Prime Minister Gordon Brown and French President Nicolas Sarkozy, in Pittsburgh for a Group of 20 summit, made a joint appearance on Friday to level the new charges against Iran.
Obama said Tehran had been building the nuclear plant in secret for years and urged it to address international concern that its nuclear program is geared toward making weapons rather than for peaceful uses.
Ahmadinejad said Iran's disclosure of the site to the IAEA on Monday was earlier than required, indicating that the plant would not begin first stages of operation for 18 months.
"It's not a secret site. If it was, why would we have informed the IAEA about it a year ahead of time?" Ahmadinejad said. "They will regret this announcement."
The Iranian leader accused the Western countries of trying to stack the table against Iran before an October 1 meeting between Iranian and Western negotiators on the nuclear issue.
"They wanted to set up a sort of media game, take the stage to sort of set up the upper hand. This is not nice," he said. "This kind of behavior goes back to what was done in the past."
He rejected Obama's description of the site as inconsistent with peaceful nuclear purposes.
"I don't think Mr Obama is a nuclear expert. We have to leave it to the IAEA and let the IAEA carry out its duty," he said. "The IAEA doesn't have a right to give an opinion, but to come and inspect and have surveillance over the facility."
While Ahmadinejad said he was hopeful about the October meeting, he stressed Iran's position that it would not negotiate away its overall right to a nuclear program.
"US, Britain and France have no business questioning Iran about its nuclear program. We are a country like other countries," he said.
He said Iran needed enriched uranium for medical purposes, and was willing to buy it from any country that would sell.
A senior US official told reporters it appeared the Iranian facility was at least a few months from having all centrifuges installed and able to operate. The nascent plant was believed to be designed for about 3,000 centrifuges for enriching uranium.
The watchdog has asked Iran to provide access to and information about the plant, built inside a mountain around 100 miles (160 km) southwest of Tehran, as soon as possible.
Ahmadinejad said Iran was not worried about suggestions that Israel - which he has in the past said should be wiped off the map - could stage a pre-emptive strike against Iran's nuclear facilities.
"We are not concerned (about an Israeli attack). Iran is a very big country. Much larger and bigger than what some people think and imagine," he said. "(Israel) wouldn't dare to attack Iran. We are capable of defending ourselves."