Tehran has blamed the attack on "regional terror sponsors and their United States masters".
Last year, Islamic State militants staged an attack in Tehran that killed 18 people. "Iran will respond swiftly and decisively in defence of Iranian lives".
Semi-official news outlets with reporters at the scene said the attackers were dressed in military fatigues and had approached the parade from a park, behind a temporary reviewing stand from where officials had been watching the ceremony.
He also said that based on preliminary reports, 4 terrorists carried out the Saturday attack in Ahwaz, adding that two of them were killed and two others were arrested. Al-Tostari said the attack undermined the Iranian government "on the day it wants to give a message to the world that it is powerful and in control".
Local media said that at least 11 members of the Islamic Revolutionary Guards (IRGC) have been killed in the incident, while Iran's state-run Islamic Republic News Agency (IRNA) reported that 20 people had been wounded.
"We will never decrease our defensive capabilities. we will increase them day by day", Mr Rouhani said at a separate military parade in the capital. However, Iran faced a bloody assault previous year from the Islamic State group, and Arab separatists in the region have attacked oil pipelines there in the past. Ahvaz is in the centre of Khuzestan province, where there have been sporadic protests by the Arab minority in mainly Shia Iran.
The attack by militants in Ahvaz in southwestern Iran Saturday resulted in 24 killed, including women and children, according to Iran's official news agency.
He blamed Saudi Arabia for fueling the movement.
Volunteers from Muskingum County aid in Hurricane Florence relief
She advised those who wish to volunteer in the Florence response efforts to call the local Red Cross office at 423-378-8700. Google is matching up to $1 million in donations to the American Red Cross to support Florence relief.
Liverpool vs Southampton, Premier League
Liverpool boss Jurgen Klopp says that midfielder James Milner is setting "the best example" to the rest of his first-team squad. I am fine with him, I think he is fine with me and the situation, so that's all.
India calls off talks with Pakistan at UNGA
Pakistan Rangers also extended help in efforts to locate the soldier's body", said the FO statement issued on Friday night. Pakistan's foreign minister Shah Mehmood Qureshi blamed Indian politics.
Saturday's attack came after a co-ordinated June 7, 2017 Islamic State group assault on parliament and the shrine of Ayatollah Ruhollah Khomeini in Tehran.
In one video posted online, soldiers participating in the parade initially seemed confused about the source of the gunfire.
Zarif did not specify which regional government he held responsible for the shooting, but Iran's elite Revolutionary Guards said the attackers were funded by Sunni arch rival Saudi Arabia.
But attacks on regime targets inside major cities are far rarer.
Russian President Vladimir Putin expressed his condolences to Iran, saying he was "appalled by this bloody crime" against Russia's ally.
It coincided with remarks by Iranian president Hassan Rouhani, who said that USA president Donald Trump will fail in his confrontation with Iran, and vowed Tehran will not abandon its missiles.
Earlier, Iran's President Hassan Rouhani used the occasion of the military parades to compare US President Donald Trump to the former Iraqi dictator Saddam Hussein, Press TV reported.
From Tehran, Rouhani watched a military parade that included ballistic missiles capable of reaching Israel and USA military bases in the Mideast.