In this photo provided by the Iranian Students' News Agency, ISNA, wounded military personnel are carried into an ambulance after a shooting during a military parade marking the 38th anniversary of Iraq's 1980 invasion of Iran, in the southwestern city of Ahvaz, Iran, Saturday, Sept. 22, 2018.
Within hours of Saturday's attack, Iran summoned diplomats from Denmark, the Netherlands, and the United Kingdom, with IRNA reporting that they were "informed of Iran's strong protests over their respective countries' hosting of some members of the terrorist group".
Three attackers were killed at the scene, he said, and the fourth died later of his injuries, he told state television.
Gunmen disguised as soldiers in military uniforms, began shooting from behind the stands during the parade, according to the state-run news agency.
In a keynote speech, Rouhani vowed to boost Iran's ballistic missile capabilities despite Western concerns that were cited by his USA counterpart Donald Trump in May when he abandoned a landmark nuclear deal with Tehran.
"Iran will respond swiftly and decisively in defense of Iranian lives", he wrote.
Iran's Foreign Minister Mohammad Javad Zarif immediately blamed the attack on regional countries and their "U.S. masters", calling the gunmen "terrorists recruited, trained armed and paid" by foreign powers.
Today's attack, which struck on Ahvaz's Quds, or Jerusalem Boulevard, comes after a coordinated June 7, 2017, Islamic State group assault on Parliament and the shrine of Ayatollah Ruhollah Khomeini, Tehran.
President Hassan Rouhani ordered Iran's Intelligence Ministry to immediately investigate the attack. We want to keep our journalism open and accessible and be able to keep providing you with news and analyses from the frontlines of Israel, the Middle East and the Jewish World.
Protests against the government and its handling of the economy that erupted across many provinces late past year also took place in some cities in Khuzestan, which reported some of the most violent protests.
A spokesman for the Revolutionary Guards, however, alleged that the attackers were separatists backed by Saudia Arabia, Iran's regional archrival. The attack targeted a military parade commemorating the beginning of the Iran-Iraq war. The Ahvaz National Resistance is one of the groups that make up the Arab Struggle Movement for the Liberation of Ahvaz (ASMLA), an umbrella group for Iranian Ahvazi opposition parties.
Ahvaz is in the centre of Khuzestan province, where there have been sporadic protests by minority Arabs. At least 18 people were killed and more than 50 wounded in the attacks.
"They are not from Daesh [ISIS] or other groups fighting [Iran's] Islamic system", Brig Gen Abolfazl Shekarchi told the official news agency IRNA.
The Islamic Revolutionary Guards Corp (IRGC) have been the sword and shield of Shi'ite clerical rule since Iran's 1979 Islamic Revolution. Iran backs militant groups across the region. In June previous year, suicide bombers attacked parliament and the mausoleum of the Islamic Republic's founder Ayatollah Khomeini, killing 18 people.
In the last decade, mass-casualty militant attacks have been incredibly rare.