Thousands of people have gathered in the streets of the southwestern Iranian city of Ahvaz to mourn the victims of an attack on a military parade that killed at least 25 people and wounded dozens.
At a September 24 ceremony attended by soldiers, clerics, and officials, mourners carried the coffins of victims wrapped in the national flag, as well as pictures of the dead and banners bearing slogans such as "No to terrorism."
Meanwhile, Supreme Leader Ayatollah Ali Khamenei reiterated accusations against the United States and other countries that have tense ties with Tehran, and said that Iran would "severely punish" those behind the attack.
"Based on reports, this cowardly act was done by people who the Americans come to help when they are trapped in Syria and Iraq, and are paid by Saudi Arabia and the U.A.E.," Khamenei was quoted as saying on his official website.
At the ceremony in Ahvaz, some mourners chanted "death to Israel and America." The United States has condemned the attack.
Trucks were seen spraying water onto the crowd as temperatures hit 40 degrees Celsius.
Authorities declared a national day of mourning, and public offices, banks, schools, and universities will remain closed in Khuzestan Province.
Iranian state media reported that four gunmen dressed in military uniforms opened fire during a September 22 parade in Ahvaz marking the start of the 1980-88 Iran-Iraq war.
State media said the four attackers were dead.
The victims of the assault included civilians and members of the elite Islamic Revolutionary Guards Corps (IRGC).
One of the victims was a 4-year-old child.
The September 24 ceremony took place on a central square in front of Sarallah Mosque and kicked off with speeches by security officials.
Mourners, mostly wearing black, beat their chests in unison, a traditional mourning practice of Shi'a, as a religious chanter sang songs of lamentation.
The coffins were later transported to the city cemetery and laid to rest.
In his address, IRGC deputy head Hossein Salami reiterated accusations against the United States and Israel and warned the two countries to expect a "crushing and devastating" response from Tehran.
Intelligence Minister Mahmud Alavi told the crowd that arrests have been made in connection with the attack.
"The terrorists themselves have perished, our agents will identify their remnants and supporters to the last man. A major part of them have already been arrested," Alavi said. He gave no number or details.
Two groups have claimed responsibility for the attack: the ethnic Arab antigovernment Ahvaz National Resistance and the extremist group Islamic State (IS).
A video released by the IS-affiliated Amaq news agency showed three men in a vehicle who appear to be dressed in IRGC uniforms. The men did not identify themselves as IS members but said they were on their way to carry out an attack on an Iranian military parade.
President Hassan Rohani and other Iranian leaders blamed the Arab separatist group for the “terrorist” attack and accused the United States, Israel, and Gulf Arab monarchies of backing it.
U.S. State Department spokeswoman Heather Nauert said late on September 22 that the United States "condemns all acts of terrorism and the loss of any innocent lives."
"We stand with the Iranian people against the scourge of radical Islamic terrorism and express our sympathy to them at this terrible time," Nauert said.
A senior United Arab Emirates official called the accusations "baseless."
Responding to Rohani’s accusations, the U.S. ambassador to the UN, Nikki Haley, told the BBC that Iran should "look in the mirror" for the causes of the attack, saying that Iran had "oppressed its people for a long time.”
Rohani and U.S. President Donald Trump are both attending the UN General Assembly in New York this week.