Protesters rallied at several points in the capital, chanting "Death to the dictator" and urging authorities to free Mir Hossein Mousavi and Mahdi Karroubi, whose family and supporters claim are now under full-time detention, according to kaleme.com.
Witnesses said riot police charged on protesters in central Tehran to try to scatter crowds. Some police took swipes at cars whose drivers were believed to be honking their horns in support of the demonstrators.
Family members and opposition activists say the two leaders have been moved from house arrest to a Tehran prison along with their wives. Iranian authorities deny the reports, but the two opposition leaders have not been seen in public or have posted statements on their websites in more than a week.
Mousavi and Karroubi were put under house arrest after they called for a Feb. 14 protest rally, the largest in more than a year following a relentless crackdown by Iranian authorities. Clashes between protesters and security forces during the demonstrations killed two and wounded dozens.
The opposition movement -- dispirited and in disarray just a month ago -- has been revived by inspiration from the Arab world's political revolts and the reports of the detentions of Karroubi and Mousavi, who claims he was rightful winner of 2009 election and that President Mahmoud Ahmadinejad was re-elected through massive vote fraud.
Both Mousavi and Karroubi -- who also ran in the June 2009 election -- have been under increasing pressure from authorities who crushed street protests by their supporters.
The protests that swept Iran after the disputed election grew into a larger movement opposed to Iran's ruling system. Hundreds of thousands peacefully took to the streets, but a heavy security crackdown crushed the protests.
Iran's leadership has rejected calls by hardliners to bring the two to trial on anti-state charges, fearing that it could serve as a rallying point for the beleaguered opposition's supporters. The current claims about their detentions, however, could also help to re-energize opposition forces.
Foreign Ministry spokesman Ramin Mehmanparast said Iran will not respond to international questions about the whereabouts of the two, adding that the country considers the matter a "completely domestic" affair.
The semiofficial news agency ISNA quoted state prosecutor Gholam Hossein Mohseni Ejehei as saying the two were not detained but did not elaborate. On Monday, he said authorities have cut all outside contact with them as part of a campaign to silence dissent.
However, the opposition and their relatives said they are being held in a military garrison in Tehran.
There has been no independent confirmation of their location.
White House spokesman Jay Carney told reporters Monday: "We obviously find the detention of opposition leaders to be unacceptable and we call on them to be treated well and released."
Mehmanparast denounced outside pressures to clarify the status of the two opposition figures.
"The internal issues of our country are completely domestic and no country is and will be allowed to interfere in the internal affairs of our country," he told reporters.
Mehmanparast said any "issues relating to" Mousavi and Karroubi "will be dealt in the framework of law by judicial authorities."