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.
Iran's foreign minister said "terrorists" backed by "a foreign regime" were behind the attack.
State media and officials have blamed Sunni militants or Arab nationalists.</p>
Fars news agency said the attack started at 09:00 local time (06:30 BST) and appeared to involve four gunmen.
The attackers fired at civilians and attempted to attack military officials on the podium, Fars reports.
Iran's Foreign Minister Mohammad Javad Zarif immediately blamed the attack on regional countries and their "U.S. masters," further raising regional tensions as Tehran's nuclear deal with world powers is in jeopardy after President Donald Trump withdrew America from the accord.
"Iran will respond swiftly and decisively in defense of Iranian lives," he wrote on Twitter.
State television aired footage of the aftermath of the assault on Ahvaz's Quds, or Jerusalem, Boulevard, which like many other places around the country saw an annual parade marking the start of Iran's long 1980s war with Iraq. The images included paramedics trying to help one person in military fatigues as other armed security personnel shouted at each other. The semi-official ISNA news agency published photographs of the attack's aftermath, with bloodied troops in dress uniforms helping each other walk away.
A local news agency in Khuzestan province, of which Ahvaz is the capital, aired grainy mobile phone footage showing parade goers fleeing as soldiers lay flat on the ground. Gunfire rang out in the background.
"Security forces have restored security in the area but the parade has totally been disrupted," a reporter on the scene for Iranian state television said by phone in a live broadcast. "People have been killed but we have no figures yet."
Zarif on Twitter said that the gunmen were "terrorists recruited, trained, armed & paid by a foreign regime." He did not immediately elaborate. However, Arab separatist groups in the region have launched attacks on oil pipelines there in the past and Iran. The separatists also accuse Iran's Shiite theocracy of discriminating against its Sunni Arab citizens. Iran has blamed its Mideast archrival, the Sunni kingdom of Saudi Arabia, for funding their activity.