Iran has promised to avenge a deadly attack on a military parade in Ahvaz that killed 25 people.
Children and 12 members of the country's elite Islamic Revolutionary Guard Corps were among the dead.
The attack started at 09:00 local time and lasted for about 10 minutes.The attackers fired at civilians and tried to attack military officials on the podium. Iranian state media says all four gunmen were killed.
The so-called Islamic State and the separatist Arab-Sunni group Al-Ahvazieh have both claimed responsibility for the attack. Tehran says the groups are financed by Saudi Arabia, Iran's Sunni rival in the region.
Ahvaz is in the centre of Khuzestan province, where there have been sporadic protests by the Arab minority in mainly Shi'ite Iran.
Foreign Minister Javad Zarif blamed "terrorists paid by a foreign regime", adding that "Iran holds regional terror sponsors and their US masters accountable".
Iran was marking the anniversary of the start of its 1980-88 war with Iraq with military parades across the nation.
The attack in Ahvaz took place on the eve of President Rohani scheduled departure for New York to attend the United Nations General Assembly.