The country's intelligence ministry said the security forces had already arrested a 'terrorist group'. The team was planning a third attack. They bruised it.
These incidents happened in less than a month after President Hassan Rouhani was re-elected.
Governor Hossein Hashemi also said that one of the attackers was shot dead and the rest were arrested.
However, it is not known whether or not there are 12 people killed in the attack.
According to lawmaker Ilias Hagar, state television website reported that three gunmen stormed the parliament in the parliament building in Central Tehran parliament. One handed one hand and a pistol and both were AK-47.
Iranian state broadcaster IRIB said there was a suicide vest on an attacker. However, some news agencies said the terrorists caused a grenade explosion.
Iran English television channel Press TV said that in about half-an-hour after the incident in the parliament, gunfire occurred in the grave of Ayatollah Khomeini, who led Iran Islamic Revolution in 1979, south of the city. Several common people were injured in this.
According to Tehran's governor, the IRIB said an attacker killed a suicide vest. Another security force was killed. Others have been arrested.