A group of four gunmen opened fire on a military parade in the southwestern Iranian city of Ahvaz in oil-rich Khuzestan province, killing at least 29 people, according to state TV.
"This terrorist attack has so far left behind 29 martyrs and 57 wounded," Mojtaba Zolnouri, a member of Iran's parliamentary national security and foreign policy commission, told Arabic language state TV al-Alam.
Iran's IRNA news agency reported earlier that half of the victims were members of the Revolutionary Guards, adding there were also civilian casualties including women and children.
The Khuzestan's deputy governor Ali-Hossein Hosseinzadeh told media that one journalist was among those killed.
But IRNA denied that a journalist died in the attack.
The aftermath of Iran military parade attack, September 22, 2018 /Reuters Photo
Saturday's deadly attack was later claimed by an anti-government Arab group, Ahvaz National Resistance, and the ISIL, both provided no evidence.
Iran's Foreign Minister Mohammad Javad Zarif blamed "a foreign regime" backed by the US for the attack and vowed to respond "swiftly and decisively."
Screenshot from Twitter
Following the attack, Iranian President Hassan Rouhani has ordered the country's security forces to identify those responsible for the attack.
"The response of the Islamic Republic of Iran to the smallest threat will be crushing," Rouhani said in a statement on his official website. "Those who give intelligence and propaganda support to these terrorists must answer for it."
Russian President Vladimir Putin also denounced the attack and sent condolences to his Iranian counterpart, meanwhile calling for deeper anti-terror cooperation between Moscow and Tehran.
What happened?
Saturday's attack happened at around 9:00 a.m. local time, 30 minutes after the ceremony started, when a group of four assailants fired at the crowd and attempted to attack military officials on the podium, Fars news agency said.
The attack reportedly lasted for around 10 minutes, and the four gunmen were all dead by now, three of whom died at the scene and one in hospital.
"Of the four terrorists, three were sent to hell at the scene, while the fourth who had been wounded and arrested went to hell moments ago due to his severe wounds," Armed forces spokesman Brigadier General Abolfazl Shekarchi told state television.
Iran was holding a number of parades in cities including capital Tehran and the port of Bandar Abbas on the Gulf to mark the start of the country's 1980-88 war with Iraq.