Five mortar shells hit northern Tehran on Sunday but there were no reported injuries in the attack, the state IRNA news agency said.
An AFP photographer on the scene said the shells appear to have been fired from an empty plot of land just northwest of Vanak Square, a major plaza in the capital.
IRNA said security forces were still investigating the incident.
The attack came just hours after Iran announced it had inked a series of security accords with Italy, whose interior minister is in Tehran for talks, including agreements to fight terrorist groups such as the Iranian opposition People's Mujahideen.
The last major explosions heard in the Iranian capital came in October, during a visit by Spanish Prime Minister Jose Maria Aznar.
Iran said mortar shells had fallen harmlessly in an empty space in the city on that occasion, while the armed opposition People's Mujahideen claimed in a statement sent to Nicosia that it had fired shells on State Security Forces' (SSF) headquarters in Vanke Square and "several security officers and commanders of the SSF were killed or wounded."
The People's Mujahideen added "ambulances were seen entering the compound" and that buildings and vehicles were damaged in the attack. The release said the armed opposition had targeted the SSF's "Barracks 12 Farvardin,” which house its "central command.”
The statement said the SSF had orchestrated a "bloody suppression of popular protests and demonstrations all over Iran" and accused it of possessing a "chain of secret detention centers and prisons."
The last clashes between student and worker protestors came in the cities of Abadan, Ajabshir and Lamerd, the statement said.
In late December, a conservative Tehran newspaper reported two days of clashes in Lamerd in which dozens of people were wounded after police fired tear gas on demonstrators, who responded by attacking and setting fire to government buildings.
However, a statement from the People's Mujahideen received at the time by AFP in Nicosia said two people died and more than 30 were hurt in the demonstration by more than 4,000 people -- TEHRAN (AFP)
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