In footage shown on Iranian television, paramedics could be seen helping a person in military uniform lying on the ground at Ahvaz's Quds, or Jerusalem, Boulevard.
"The terrorists disguised as Islamic Revolution Guards Corps (IRGC) and Basiji (volunteer) forces opened fire to the authority and people from behind the stand during the parade", Governor of Khuzestan Gholam-Reza Shariati said, according to the state news agency IRNA.
The spokesman accused Saudi Arabia, Iran's regional rival, ofsupporting the group.
State television showed images of the immediate aftermath.
Samsung unveils first triple camera smartphone Galaxy A7 (2018)
Also, this is Samsung's first device in the smartphone market which comes with a side-mounted fingerprint sensor. Under the hood, the device comes powered by an unspecified octa-core processor clocked at 2.2GHz.
The state-run IRNA news agency said over 20 people had been wounded, including a woman and a child. Ahvaz is in the centre of Khuzestan province, which has been the site of sporadic protests by Iran's Arab minority.
The report called the attackers "Takifiri gunmen", a description that has previously been used for members of the "Islamic State" (IS) group.
The bloodshed struck a blow to security in OPEC oil producer Iran, which has been relatively stable compared with neighboring Arab countries that have grappled with upheaval since the 2011 uprisings across the Middle East.
Iran also suffered major attacks by IS extremists on June 7, 2017, when militants assaulted the parliament and the shrine of Ayatollah Ruhollah Khomeini in the capital, Tehran.
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. The assault shocked Tehran, which largely has avoided militant attacks in the decades after the tumult surrounding the Islamic Revolution.