At least eight members of Iran's elite military have been killed with 20 people wounded in a gun attack during a parade in the southwestern city of Ahvaz, local media have said.
IRNA reported those wounded in the attack included a woman and a child.
At first, those present appear to think that the gunfire could be part of the ceremony, but then quickly realize it is a hostile attack and immediately begin to scramble from the scene.
In this photo provided by the Iranian Students' News Agency, Revolutionary Guard members carry a wounded comrade after a shooting during a parade in Ahvaz, Iran, on September 22.
The attack struck on Ahvaz's Quds, or Jerusalem, Boulevard.
There was no immediate claim of responsibility.
Iran's Foreign Minister Mohammad Javad Zarif blamed a "foreign regime" backed by the United States for paying, recruiting, and training terrorists for the assault in Ahvaz.
Iranian state television said the assault, which killed 24 and wounded more than 60 people, targeted a stand where officials were gathered to watch an annual event marking the start of the Islamic Republic's 1980-88 war with Iraq.
Iranian state television quoted a spokesman for Iran's Revolutionary Guard Corps as saying that the attackers were affiliated with an Arab separatist group in the region.
He said that children and journalists were casualties in the attack. However, those groups in the past previously have only attacked unguarded oil pipelines at night. The province has a large ethnic Arab community, many of them Sunni, the Muslim denomination to which IS adheres and which is a minority in Shiite Iran. The assault shocked Tehran, which largely has avoided militant attacks in the decades after the tumult surrounding the Islamic Revolution.
Ahvaz is the capital of Iran's oil-rich Khuzestan Province.