Gunmen opened fire on a military parade in southwestern Iran, killing at least 24 people and wounding more than 53, according to state news media.
The Islamic State terror group has claimed responsibility for the attack Saturday in Ahvaz in Khuzestan province.
There are conflicting reports giving varying details.
Officials said two gunmen were killed. However, other officials indicated that two others had been arrested in connection with the attack.
State television said the assailants targeted a stand where Iranian officials had gathered to watch an annual parade commemorating the launch of the Islamic Republic's 1980-1988 war against Iraq. Similar parades were held across the country.
Most of the dead were said to be members of Iran's elite Revolutionary Guard. Some reports said one of the dead was a journalist.
In a tweet, Iranian Foreign Minister Mohammad Javad Zarif blamed the attack on 'regional terror sponsors' and 'their U.S. masters.'
The 'regional terror sponsors' is language that usually refers to Iran's arch enemies, Saudi Arabia and Israel.
Zarif said Iran would retaliate 'swiftly and decisively' for the attack.