Dressed in mourning black, they beat their chests with bare hands and lashed their shoulders with iron chains - rituals meant to show their grief over the death of Imam Hossein (AS), grandson of Islam's Prophet Mohammad (PBUH) and third Imam.
Known as Ashoura and observed by Shiites across the world, the occasion climaxed Wednesday, the 10th day of the lunar Muslim month of Moharram.
Scores of men - moved by their grief - wept as they marched in Tehran, capital of Shiite Iran. Women watched from sidewalks and balconies, as the marchers filled the streets, led by young men carrying black, green and red banners. Similar processions were staged in other cities across this nation of 70 million people.
Imam Hossein (AS) was martyred in the 680 A.D. battle fought on the plains outside Karbala, a city in modern Iraq that's home to the Imam's holy shrine and where 2 million Shiites gathered Wednesday to mark Ashoura.
In the battle, Imam Hossein (AS) was decapitated and his body mutilated by Yazid's armies. All of Imam Hossein's male family members, relatives, friends, soldiers who all together formed a 72-member army were beheaded in an unequal war with a 30,000-strong army of the enemy in the desert of Karbala.
The Imam and 72 of his loyal friends and followers were massacred by the troops of the Caliph in 680 at a place close to what is now Karbala, Iraq. The enemy army even killed his 6-month-old son after days of thirst.
In Tehran's southern neighborhoods, actors clad in armor re-enacted the Karbala battle, with performers on horseback portraying Imam Hossein (AS) and his companions dressed in green and white. Actors playing their enemies wore red.
Some are likening the current fighting in Gaza to Ashoura, saying Palestinians there, like Imam Hossein (AS) and his followers, are facing a mighty army while denied the means to fight a fair battle. So far, at least 670 Palestinians, many of whom are women and children, have been killed in the Israeli attacks. More than 2,950 others have been wounded since Israel's Gaza offensive began Dec. 27.
"The resistance of the Gaza people against the well-armed Zionist regime is derived from the lesson of Ashoura," said lawmaker Hashmatollah Felahatpisheh.
The occasion is the source of an enduring moral lesson for the Shiites.
"I'm attending the mourning ceremonies to show my love for Imam Hossein (AS)," said a weepy Siavosh Najafi, a mourner in his 30s, as he beat his chest.
"He sacrificed his blood to teach us not to give in to corruption, coercion, or use of force and to seek honor and justice."
Some bystanders in Tehran Wednesday offered sweetened water to mourners. Songs eulogizing Imam Hossein (AS) played over loudspeakers. Imam Hossein (AS) and his companions were denied water by their enemies who controlled the nearby Euphrates.
Some Tehran streets were partially covered with blood from the slaughter of hundreds of cows and sheep. Volunteers cooked the meat and fed it to the poor.
Imam Hossein's martyrdom - recounted through a rich body of prose, poetry and song - remains an inspirational example of sacrifice to Shiites, who make up about 10 percent of the world's estimated 1.3 billion Muslims.