An unknown group of assailants have set fire to a Principlist campaign center of Iranian presidential hopeful Mir-Hossein Mousavi in the holy city of Qom, Press TV reported.
"Around an hour ago, a group of unidentified men attacked the open air facility and torched it," one of the campaign activists told IRNA on Monday afternoon.
According to the unnamed official, a large number of banners and posters burnt in the fire.
As of yet, no group has claimed responsibility for the attack.
Although backed by the country's leading Reformist figure, former president Mohammad Khatami, Mir-Hossein Mousavi also enjoys support among some factions of the rival Principlist camp.
Mousavi, who served as Iran's last prime minister from 1981 to 1989, promises 'reforms and a return to the early principles of the Islamic Republic' as a campaign motto.
In many circles, the ex-premier is regarded as the main rival of incumbent Principlist President Mahmoud Ahmadinejad, who is running for a second term in office.
With Iran's tenth presidential elections just days away, this is the second time that an attack has rocked one of the candidates' campaign offices.
On Friday, unidentified gunmen opened fire on Ahmadinejad's campaign center in the southeastern city of Zahedan, injuring two adults and a two-year old in the attack.
Situated close to Iran's volatile eastern borders with Pakistan and Afghanistan, Zahedan has been the scene of several violent attacks over the years, carried out primarily by armed separatist rebels or drug traffickers.
Qom, however, is a central city situated 130 kilometers to the south of the capital Tehran.