Iran's main opposition website reported on Monday that Iran's two key opposition leaders were arrested.
The Kaleme.com site says Mir Hossein Mousavi, Mehdi Karroubi and their wives were jailed in Tehran.
Earlier on Monday, Iran acknowledged that the men and some members of their families were under government detention at an undisclosed location.
On Sunday, the International Campaign for Human Rights in Iran said the two opposition leaders were in grave danger after Iranian security forces apparently abducted them from their homes and moved them to a "safe house."
The New York-based group says that in the Iranian context, a "safe house'' is often a place where detainees are mistreated by the Revolutionary Guard to extract confessions without any judiciary oversight.
The two opposition leaders have been under house arrest. They were cut off from their families after their supporters staged an anti-government protest in Tehran on February 14. It was the largest by the opposition in more than a year.
The two men also led large-scale protests in 2009 after losing the presidential election to President Mahmoud Ahmadinejad in a vote they say was rigged. Forces loyal to Iran's government crushed those demonstrations.
On Monday, U.S. Secretary of State Hillary Clinton harshly criticized the Iranian government for waging a systematic crackdown on government opponents.
Speaking at a U.N. human rights meeting in Geneva, Clinton called for transparency from Iran on alleged human rights violations.
The United States on Sunday accused Iran of carrying out an organized campaign of intimidation and arrests of political figures, journalists, student leaders and activists.
A statement from President Barack Obama's National Security Council said Iran must stop the "blatant violation of the universal rights of its citizens and its ongoing hypocrisy."
Some information for this report was provided by AP and AFP.