Under the red flag of martyrdom, they beat their chests in memory of a fallen religious leader as the cleric recounts his fate outside one of Tehran's oldest mosques.
Imam Hussein was tricked and martyred by his enemies in the seventh-century battle of Karbala. The crowd of grown men and women wept with grief as Hussein's story was retold on Sunday.
Ashura is always deeply moving for the Shia faithful but this year even more so. It comes after the trauma of Israel's surprise attacks on Iran.
There was a sense of emotional release and a chance for Iranians to come together in solidarity.
Ashura is also a reminder that Iran's revolutionary leaders draw much of their power from the strength of religion in this country after a conflict its enemies hoped would see those same leaders toppled.
The festival has come at just the right time for its embattled government.
Iran's supreme leader has appeared in public for the first time since Israel attacked his country. Ayatollah Ali Khamenei was greeted with ecstatic cheers by his followers at Ashura prayers.
His supporters told us they welcomed his return. "I was so happy that I didn't know what to do," said one woman. "This caused our big enemies the United States and Israel to receive a great slap in the mouth."
"His appearance on TV for Ashura," a young man told us, "showed that all the talk about him hiding and taking the path of peace with the United States is not true and it shows that he is holding his position strongly and steadfastly".
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Defiance in Tehran as Khamenei makes appearance
We had been given rare access to Iran among a handful of journalists who were let in after the 12-day war.
Its scars aren't hard to find - buildings left with gaping holes where Israeli airstrikes took out members of Iran's elite, one after another.
And Abbas Aslani, an analyst with close ties to the government, says there is a fear it may not be over.
"The Iranian government and the army are prepared for a new round of conflict, because they think that the other party, specifically Israel, is not to be trusted in terms of any ceasefire," he said.
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At the Ashura ceremony, the crowd chants, "we'll never yield to humiliation" - an age-old message for Iran's enemies today as they brace for the possibility of more conflict.
(c) Sky News 2025: 'We'll never yield': Millions of Iranians unite in mourning
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