One person is known to have died in Tehran after shots were fired on an angry crowd protesting at the re-election of President Mahmoud Ahmadinejad.
It's thought the shots were fired by a volunteer militia connected to the Revolutionary Guard – a branch of the Iranian military.
Hundreds of thousands of people had defied a ban and took to the streets of the capital, claiming the presidential election was rigged.
It's the biggest public demonstration in the Islamic republic's 30-year history.
Iran’s supreme religious leader Ayatollah Khomeini has ordered an inquiry into allegations of election fraud.
Meir Javedanfar, the Director for the Middle East Economic and Political Analysis Company in Washington, told RT that Ahmadinejad may lose office, if Khomeini chooses so.