Gunmen have opened fire on an Iranian military parade in the south-western city of Ahvaz, killing at least 24 people, including civilians, and injuring more than 50, state media say.
The attackers shot from a park near the parade and were wearing military uniforms, reports say.
An anti-government Arab group, Ahvaz National Resistance, claimed the attack.
The foreign minister has blamed “terrorists paid by a foreign regime”.
Iran is marking the anniversary of the beginning of the 1980-88 war with Iraq with several military parades across the nation.
Fars news agency said the attack started at 09:00 local time (06:30 BST), lasted about 10 minutes, and appeared to involve four gunmen.
The attackers fired at civilians and attempted to attack military officials on the podium, Fars reports.
Eight members of Iran’s Revolutionary Guard and a journalist are among those killed, reports say.
“There are a number of non-military victims, including women and children who had come to watch the parade,” Irna news agency said.
Two of the gunmen were killed by security forces, while the other two were arrested, according to the local deputy governor, Ali Hosein Hoseinzadeh.
Who is behind the attack?
There have been conflicting reports.
A spokesman for the Ahvaz National Resistance, an umbrella group that claims to defend the rights of the Arab minority in Khuzestan, said the group was behind the attack.
The spokesman did not say whether the group had links to other countries.
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However, Foreign Minister Mohammad Javad Zarif said the “terrorists” behind the attack were “armed and paid by a foreign regime”, adding that “Iran holds regional terror sponsors and their US masters accountable.”
Iran has previously accused its regional rival, Saudi Arabia, of supporting separatist activity amongst Iran’s Arab minority.
Meanwhile, an Iranian military spokesman told state media the attackers were not from the Islamic State militant group, but “were trained and organised by two Gulf countries”, and had ties to the US and Israel.