At least seven people were killed at a mass rally held by tens of thousands of supporters of Iran's defeated reformist presidential candidate Mir-Hossein Mousavi, the website of local English-language Press TV reported on Tuesday.

The protesters were killed after they tried to attack a military post near Tehran's Azadi (Liberty) Square on Monday, the report said.

Several people were also wounded when protesters were trying to destroy public properties, the report added.

Press TV reported on Monday that gunshots were heard at the rally and people were running away.

"There has been sporadic shooting out there," a reporter of Press TV said, "I can see people running here."

The rally near Tehran's Azadi Square has been declared "illegal" by the authorities.

The backers of Mousavi were "staging a calm and peaceful demonstration," the Press TV reporter said earlier, adding that the anti-riot police were calm but alert standing on the sides of the street observing the surge of the people.

According to Press TV, Iran's police chief Esmail Ahmadi-Moghaddam also warned on Monday that any illegal rally would be confronted in a legal way by the police.