Bombers have targeted Iran's parliament and the shrine of the country's revolutionary founder Ruhollah Khomeini.
There are conflicting reports on the number of dead, with an official saying at least 12 people have lost their lives.
Four attackers have been killed and the fate of four reported hostages is unclear.
Attackers stormed the parliament building in Tehran, armed with guns, and one of them is believed to have blown himself up on the fourth floor during the siege.
Television footage showed members of parliament continuing routine business as the gunshots were reported in the offices around them.
In a co-ordinated attack on the mausoleum of revolutionary leader Ayatollah Khomeini in the south of the city, a female suicide bomber blew herself up as more armed assailants entered the grounds.
A gardener was killed and several others were injured, while another woman was arrested carrying grenades.
Tehran is reportedly in lockdown as police block the streets and shut down the metro.
Islamic State has said it was behind the attacks. The terror group has previously targeted the country over its efforts with Iraq and Syria's Bashar al Assad in battling them.
IS has been losing ground in both countries.
