They gathered beneath the blistering sun to show their willingness to confront the enemy, the Islamic State of Iraq and Syria and its backers. And to many Iranians, a key ally of the radical Sunni movement is another opponent of Iran: the United States.
"We are ready to fight in Iraq," said Mohammad Mehdi Sadeghi, 21, who said he wants to be a diplomat but was volunteering his services to do battle in neighboring Iraq against the forces of ISIS, the Al Qaeda breakaway faction making territorial gains in Iraq and Syria to the great consternation of both U.S. and Iranian officials. "We are here to show our power to the media and to make people vigilant."
He was among about 500 people who gathered Tuesday in the sweltering heat of Iman Hussain Square in southeast Tehran to demonstrate their determination to go to Iraq to fight ISIS if needed. It was a well-orchestrated event, with the press invited, meant to demonstrate Iranian resolve on behalf of the Iraqi government of Prime Minister Nouri Maliki, Tehran's close ally.
ISIS, an offshoot of Al Qaeda, a Sunni movement, has vowed to topple the Shiite Muslim-led government in Baghdad and attack sacred Shiite shrines in Iraq. In Iran, an overwhelmingly Shiite nation with a theocratic governing structure, the threats have raised alarms and calls for the faithful to go to Iraq.
However, would-be fighters who showed up at the square on Tuesday stressed that they were still awaiting a fatwa, or religious edict, from Iran's supreme leader, Ayatollah Ali Khamenei, directing them to go to Iraq to shield the shrines of Shiite Islam. Khamenei has yet to issue such an edict.
Still, the prospect of Al Qaeda-linked militants bearing down on Shiite holy places has stirred profound emotions here, as it has among Shiites worldwide. Iraq is home to some of the most sacred Shiite sites.
Among those gathered at the square Tuesday were men and women wearing white shrouds bearing the phrase, "We are the defenders of the holy shrines of the Shiites."
Both the U.S. and Iran view Al Qaeda-style militants such as those of ISIS as mortal enemies. There has even been some talk of the longtime rivals collaborating against the militants in Iraq. But the threat of Sunni extremism is viewed through very distinct and often clashing frames of references in the two nations.
The official Iranian press has generally presented ISIS as a creation of the United States and its allies, including Israel and Saudi Arabia, the preeminent Sunni power in the Persian Gulf and an arch-rival of Iran. Iranian analysts have described ISIS's emergence in Syria and Iraq as part of a broader, U.S.-backed strategy to weaken Iran and its regional allies, including the governments of Iraq and Syria.
"ISIS is the hirelings of America, Saudi Arabia, Qatar and Israel," said Sadeghi as he fingered worry beads. "This is all about providing a pretext for a return of American forces to Iraq. As our supreme leader has said, the U.S.A. is of no help to any country in the region."
Some Iranian volunteers have already gone to Syria to fight on behalf of the government of President Bashar Assad. A number of Iranian men have been killed in Syria, including some military officers. Inevitably, they have been described as having lost their lives defending the shrine of Sayyida Zainab, a revered Shiite monument outside Damascus.
"We pray to God that our military assistance is not needed in Iraq," said Mahnaz Amiri, 46, sitting in the square with her 6-year-old son. "But if the supreme leader decides that jihad against ISIS is needed, then we will follow his instructions."