The claims by the National Council of Resistance of Iran (NCRI) came the day before the UN nuclear watchdog is to meet in Vienna Thursday to consider a report that Iran hid nuclear activities for 18 years, including the making of small amounts of plutonium and enriched uranium.
The United States charges that Iran is secretely developing nuclear weapons, but International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei said in his report the agency has so far found no evidence that Iran is trying to make the bomb.
NCRI spokesman Firouz Mahvi told reporters that in one incident in eight months of inspections, IAEA inspectors were looking for a workshop for nuclear activities and Iranian authorities took them "to a site in Hashtgerd, around Karaj," 40 kilometres (25 miles) west of Tehran, to show them there was no workshop.
Mahvi said: "Information from within the clerical regime made it clear that they had been taken to a site, similar to the site in question and that they were not shown the actual site.
"This is one example of the regime's deceptive tactics," Mahvi said.
He did not provide evidence to back up his statement and IAEA officials refused comment.
Mahvi said Iran was now cooperating with the IAEA but hiding what it can since it wants to "buy time" in order to get to the "point of no return where no matter what you do (against them) they can get nuclear weapons."
The NCRI had in August 2002 blew the whistle on secret Iranian activities when it revealed that the Islamic regime was building an underground uranium-enrichment plant at Natanz, 250 kilometres (150 miles) south of Tehran, and a heavy-water production facility at Arak, about 200 kilometres southwest of Tehran.
Iran eventually declared both these sites to the IAEA, leading to ElBaradei's visit in February 2003 to Tehran.
The IAEA has carried out eight months of inspections since then, finally increasing pressure on Iran by imposing an October 31 deadline for Tehran to fully disclose its nuclear program.
The NCRI considers itself a government in exile but the US State Department has listed it and its armed wing, the People's Mujahedeen as a terrorist organization.