Gunmen attacked a military parade in the southwestern Iranian city of Ahvaz on Saturday, killing 24 and wounding 53, Iran's state-run IRNA news agency said.
Iran is marking the anniversary of the beginning of the 1980-88 war with Iraq with several parades across the nation.
Kurdish militants killed 10 Revolutionary Guards in an attack on an IRGC post on the Iraqi border in July, Iran's semi-official Tasnim news agency reported, the latest bloodshed in an area where armed Kurdish opposition groups are active.
Iran holds "regional terror sponsors" and their "US masters" responsible for the attack, he said.
State television blamed "takfiri elements", a reference to Sunni militants, for the attack. He said children and journalists were casualties in the attack.
Tensions between traditional rivals Iran and Saudi Arabia have surged in recent years, with the two countries supporting opposite sides in wars in Syria and Yemen and rival political parties in Iraq and Lebanon. The images included paramedics trying to help one person in military fatigues as other armed security personnel shouted at each other.
Who carried out the assault also remained in question.
The report called the attackers "Takifiri gunmen", a description that has previously been used for members of the "Islamic State" (IS) group. The Guard also has vast holdings in Iran's economy.
Iran is also now facing intensified attacks in its Kurdish-majority provinces (Rojhilat) and along its Western border with the Kurdistan Region, where Kurdish opposition groups operate.
Saturday's attack comes after a coordinated Islamic State assault on parliament and the shrine of Ayatollah Ruhollah Khomeini in Teheran in June previous year. The province in the past has seen Arab separatists attack oil pipelines. At least 18 people were killed and more than 50 wounded in the attacks.