Thousands of people packed the streets of the southwestern Iranian city of Ahvaz to mourn the victims.
Mourners, mostly wearing black, beat their chests in unison, a traditional mourning practice of Shi'a, as a religious chanter sang songs of lamentation. Mourners played drums, cymbals and horns, according to local custom.
As he spoke the crowd chanted, "Death to America" and "Death to Israel".
On June 7, 2017 in Tehran, 17 people were killed and dozens wounded in simultaneous attacks on the parliament and on the tomb of revolutionary leader Ayatollah Ruhollah Khomeini - the first inside Iran claimed by IS.
Monjazi was in the wheelchair watching the parade when the gunshots erupted and was unable to find shelter from the hail of bullets. Many held pictures of a four-year-old boy killed in the incident, one of the worst such attacks against Iran's most powerful military force.
Speaking before a funeral for one of the victims on Monday, the Revolutionary Guards' deputy head Hossein Salami warned the USA and Israel in a speech broadcast on state television.
There has been no case that "terrorists do an action and not receive a response", he said.
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"We warn all of those behind the story", said Salami, "we will take revenge".
Khamenei, who was speaking to a group of Iranian athletes, said the attack "once again shows the Iranian nation faces many enemies on its proud path of progress and development". "The terrorists are a bunch of cowards".
"Just look at the crowd, with no fear, people are gathered here to see their soldiers and martyrs off to heaven", said another Ahvaz resident, Ghaseem Farhani, a merchant. As some soldiers crawled about as gunfire rang out, others shielded civilians while women and children fled for their lives.
In a further claim, Yaghub Hur Totsari, a spokesman for the Arab Struggle Movement to Liberate Ahvaz, told Reuters the Ahvaz National Resistance umbrella organization of Arab anti-government armed movements was behind the attack, but did not specify which particular group carried it out.
President Hassan Rouhani on Sunday accused an unnamed USA -allied regional country of supporting the perpetrators.
Iran's Intelligence Minister Mahmoud Alawi announced that the country's security forces managed to detain members of a "terrorist team" behind the recent deadly attack in the southwestern city of Ahvaz.
But Nikki Haley, the USA ambassador to the United Nations, said Iran should "look in the mirror" for the causes of the attack and that Rouhani had "oppressed his people for a long time".
Analysts said the violence has led to a boost in domestic support for the Guards which they could use to silence their critics, who include pragmatic President Hassan Rouhani.