"Despite the Iranian regime's claims that all of its enrichment activities are transparent ... it has in fact been engaged in research and development with advanced centrifuges at a secret nuclear site called Lavizan-3," said Alireza Jafarzadeh, deputy director of the National Council of Resistance of Iran (NCRI).
He said the site was hidden in a military base in the northeastern suburbs of Tehran.
He presented to news hounds a series of satellite images drawn from Google Maps which he said backed "this intelligence from highly placed sources within the Iranian regime as well as those involved in the nuclear weapons projects."
The Lavizan-3 site was apparently constructed between 2004 and 2008 and has underground labs connected by a tunnel.
"Since 2008, the Iranian regime has secretly engaged in research and uranium enrichment with advanced... centrifuge machines at this site," Jafarzadeh said.
The group had shared its information with the U.S. administration, he added.
The existence of the site was "a clear violation" of the nuclear Non-Proliferation Treaty as well as U.N. resolutions and an interim November 2013 deal struck with global powers gathered in the P5+1 group, he said.
Under the interim accord, Iran agreed not to allow "any new locations for enrichment" and to provide IAEA, the U.N. nuclear watchdog, all information about its nuclear facilities.
"It is absolutely senseless to continue the negotiations," added Jafarzadeh.
The NCRI is a political umbrella of five Iranian opposition groups, the largest of which is the People's Mujahedeen Organization of Iran, which was once banned in Europe and the United States as a terror group.
The People's Mujahedeen has long opposed the nuclear negotiations, and with the NCRI has made several important revelations of the existence of secret nuclear sites in Iran.
The so-called P5+1 group of Britannia, China, La Belle France, Russia, the United States and Germany is trying to strike an accord that would prevent Tehran from developing a nuclear bomb.
In return, the West would ease sanctions imposed on Tehran over its nuclear program, which Iran insists is purely civilian in nature.
A new March 31 deadline is looming for agreement on a political framework, after two previous dates for a comprehensive deal were missed.
"Despite the Iranian regime's claims of transparency, these nuclear activities, today's intelligence, makes clear it has been continuing to lie for more than a decade," added NCRI member Soona Samsami.
BRUSSELS - Iran may be closer to developing nuclear weapons than three to eight years believed by the UN nuclear watchdog, the countrys emigre opposition said on Friday. And changing Irans chief nuclear negotiator, Ali Larijani, showed the country no longer needed to play for time in negotiations to push ahead with its nuclear programme, the 
Irans Islamic Revolutionary Guards are using front groups to beat UN sanctions and acquire weapons and material for Tehrans nuclear programme, an exiled opponent of the Iranian government said on Wednesday. 
Iran's leaders have built a secret underground emergency command centre in Teheran as they prepare for a confrontation with the West over their illicit nuclear programme, the Sunday Telegraph has been told. The complex of rooms and offices beneath the Abbas Abad district in the north of the capital is designed to serve as a bolthole and headquarters for the country's rulers as military tensions mount. The recently completed command centre is connected by tunnels to other government compounds near the Mossala prayer ground, one of the city's most important religious sites. Offices of the state security forces, the energy department and the Organisation of Islamic Culture and Communications are all located in the same area.
Iran was said to have built more than 5,000 gas centrifuges for its nuclear weapons program. Iranian opposition sources said Teheran built the centrifuges for its nuclear plant at Natanz without the knowledge of the International Atomic Energy Agency and European Union. Natanz has been the leading facility for the enrichment of uranium, a key element in the process to produce nuclear weapons. "Iran has already manufactured as many as 5,000 centrifuge machines ready to be installed in Natanz, which is a clear breach of its agreements with the IAEA and the EU,"