A group of gunmen opened fire on a military parade in the Iranian city of Ahvaz on Saturday, killing several people and wounding at least 20 more, local and state news agencies reported.
State television showed images of the immediate aftermath of the attack in Ahvaz.
It has been reported that two of the assailents have been killed and two others arrested, Mehr reported.
"Individuals disguised in the Islamic Revolution Guards Corps and Basij uniforms fired at officials and people from behind the stand, leaving a number of innocent people including women and children martyred or injured", Governor of Khuzestan Province Gholamreza Shariati said.
"Iran holds regional terror sponsors and their USA masters accountable for such attacks" Zarif wrote in a tweet on Saturday.
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"Security forces have restored security in the area but the parade has totally been disrupted", a reporter on the scene for Iranian state television said by phone in a live broadcast. He said children and journalists were casualties in the attack.
Early reports Saturday described the assailants as "Takifiri gunmen", a term previously used to describe the Islamic State group. The Guards also play a major role in Iran's regional interests in countries such as Iraq, Syria and Yemen. Paramedics could be seen helping someone in military fatigues lying on the ground.
The parade in the southwestern city was being held to mark the anniversary of the start of the eight-year war with Saddam Hussein's Iraq in 1980. At least 18 people were killed and more than 50 wounded.
Ahvaz is the capital of Khuzestan province, an oil-rich region where Arab separatists have been known to attack oil pipelines in the past.
Khomeini led the 1979 Islamic Revolution that toppled the Western-backed shah to become Iran's first supreme leader until his death in 1989.