At least 24 people died and 53 were injured Saturday when gunmen disguised as soldiers opened fire on an elite military parade in the southwestern Iranian city of Ahvaz.
State media said at least 24 people, including civilians, soldiers, a woman, and a child, died when at least four assailants dressed in Iranian military attire opened fire on the parade from a nearby park in an assault that lasted around 10 minutes.
"Terrorists recruited, trained, armed and paid by a foreign regime have attacked Ahvaz," Javad Zarif, Iran's foreign minister, said in a tweet.
"Children and journos among the casualties. Iran holds regional terror sponsors and their US masters accountable for such attacks. Iran will respond swiftly and decisively in defense of Iranian lives," he added.
The Islamic Republic New Agency blamed Arab nationalist groups or extremists for the assault.
Two of the alleged terrorists were killed by security forces while two others were detained, according to the same sources.
Iran's regime was staging military parades across the country in commemoration of the Iran-Iraq war (1980-88).
Ahvaz, located near the Iraqi border in Khuzestan province, is a sprawling city that was devastated during the conflict.
Elite Islamic Revolutionary Guard Corps were present at the Ahvaz military parade.
Attacks inside Iran are not common, even though extremist groups are known to engage the troops near its borders with Pakistan and Iraq.
The last major terror attack in the country occurred in June 2017 on the Iranian parliament in Tehran, which was claimed by the Islamic State terror organization, and in which 17 people died and more than 50 were wounded.