Gunmen have attacked a military parade in the south-west Iranian city of Ahvaz, killing at least eight members of the Revolutionary Guard and wounding 20 people, state media said.
"The number of martyrs has reached 11 and could rise further, and the wounded are now more than 30", the semi-official ISNA news agency quoted Khuzestan deputy governor Ali-Hossein Hosseinzadeh as saying.
The semi-official ISNA news agency published photographs of the attack's aftermath, with bloodied troops in dress uniforms helping each other walk away.
State television blamed "takfiri elements", a reference to Sunni militants, for the attack.
The attack in Ahvaz came as Iranian President Hassan Rouhani was among the dignitaries at the main anniversary parade in Tehran.
There was no immediate claim of responsibility for the attack in the city of Ahvaz.
Map of Iran locating Ahvaz
Mojtaba Zonnour, a member of the Parliament's National Security and Foreign Policy Commission, told Tehran-based al-Alam News Network that the number of the martyred of a Saturday terrorist attack at a military parade in Ahvaz has risen to 29, with 57 injured, including women and children.
Foreign Minister Mohammed Javad Zarif blamed the bloodshed on "regional terror sponsors", language that usually refers to Iran's enemies Saudi Arabia and Israel, and "their United States masters", and vowed that Tehran would respond decisively.
He said that children and journalists were casualties in the attack.
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.
He added that the gunmen were "terrorists recruited, trained, armed and paid by a foreign regime". Khomeini led the 1979 Islamic Revolution that toppled the Western-backed shah to become Iran's first supreme leader until his death in 1989.
Ahvaz is the capital of Khuzestan province, an oil-rich region where Arab separatists have been known to attack oil pipelines in the past. The assault shocked Tehran, which has seen few militant attacks in the decades after the tumult surrounding the Islamic Revolution.
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