|Iran will not respond to international questions about the whereabouts of Mousavi, left, and Karroubi [AFP]
Iran's security forces fired teargas and clashed with opposition supporters in central Tehran, where demonstrators were rallying to demand the release of two opposition leaders, according to witnesses and an opposition website.
"Security forces and people in civilian clothes clashed with demonstrators in Tehran to disperse them," opposition website Kaleme reported about Tuesday's events.
Kaleme.com said protesters rallied at several points in the capital, chanting "death to the dictator" and calling on authorities to free Mir Hossein Mousavi and Mahdi Karroubi.
Former presidential candidates Mousavi and Karoubi have not been seen in public since they called for a rally on February 14, during which thousands of their supporters took to the streets to support uprisings in the Arab world.
Family members and opposition activists say the two leaders have moved from house arrest to a Tehran prison along with their wives. Iranian authorities deny the reports.
Sahamnews, another opposition website, reported large numbers of security personnel stationed at main streets and squares in Tehran "to prevent gathering of opposition supporters."
Iran has said it will not respond to international questions about the whereabouts of the opposition leaders.
Tehran considers the matter a "completely domestic" affair, said Ramin Mehmanparast, the foreign ministry spokesman .
Mehmanparast said any "issues relating to" Mousavi and Karroubi "will be dealt in the framework of law by judicial authorities".
The semiofficial news agency ISNA quoted prosecutor Gholam Hossein Mohseni Ejehi as saying that the two men are not in a prison, but the report gave no further details.
Mousavi and Karroubi's suspected detention comes after the country's former reformist president, Mohammad Khatami, called on the authorities to release them.
Karroubi, Mousavi and their wives were under house arrest and living in complete isolation, their homes under surveillance and cut off from the outside world, according to their websites.
Calls for protests
The February 14 protests, the first to be held since February last year, angered the authorities who accused Mousavi and Karroubi of treason, according to opposition websites.
Mousavi and Karroubi had previously led a string of protests in Iran after president Mahmoud Ahmadinejad's controversial re-election in June 2009, which they claim was rigged.
Meanwhile, calls continued on social media sites for opposition rallies in Tehran, where security forces and plainclothes militiamen were deployed in efforts to stamp out any protest gatherings.
On Tuesday, witnesses said several people were arrested after police fired teargas on demonstrators in central Tehran.
The atmosphere was reportedly "volatile" in Valiasr Square and street and Enqelab Street in Tehran, the Iran Students Committe said. "Security forces and riot police are stationed all along the streets and intersections," the group said in a statement.
Clashes between protesters and security forces during the recent demonstrations have killed two and wounded dozens.