To the sound of chanting, drums, and the ritual beating of the chest with palms -- or, for some, the back with chains -- the pilgrims are observing Arba'in, a word meaning "the 40th day since an event."
The pilgrims are marking the 40th day of mourning for Imam Husayn, a key Shi'ite figure whose death in a seventh-century battle sealed the schism dividing Islam into its Shi'ite and Sunni branches.
Heavy Security Presence
Guarding the huge crowds are some 10,000 security forces.
"With regards to security measures, we have stepped up security inside and outside the province," police officer Razzaq Musa told Reuters. "We are searching all the pilgrims who come to mark the Arba'in of Imam al-Husayn. We are in full control of the province."
Early on March 19, three small missiles or mortar rounds were fired into the city but there were no casualties.
Officials worry about a repeat of suicide bombings and other attacks that killed 171 people in Karbala and Baghdad during the same Shi'ite observance on March 2, 2004.
Fears of violence are also high because this year's observance closely follows the bombing of a key Shi'ite shrine in Samarra on February 22. That provoked waves of tit-for-tat sectarian killings that have claimed some 500 lives.
Can Civil War Be Avoided?
Amid such tensions, Iraq is today marking its third anniversary since the U.S.-led invasion began overnight on March 19-20. Official comments on the anniversary have largely focused on questions of whether sectarian attacks are pushing Iraq toward civil war.
Former interim Prime Minister Iyad Allawi told BBC television that Iraq is nearing "the point of no return" and has already plunged into sectarian civil war.
But U.S. officials say there is no such imminent danger. Vice President Dick Cheney told U.S. CBS television that "terrorist" groups have tried to "foment civil war but I don't think they've been successful."
U.S. and Iraqi forces are currently sweeping the area around Samarra, north of Baghdad, for insurgents.
Meanwhile, U.S. President George W. Bush said on March 19 that Washington regards success in Iraq as essential to making America itself secure in the war on terror.
"We are implementing a strategy that will lead to victory in Iraq," Bush said. "And a victory in Iraq will make this country [the United States] more secure and will help lay the foundation of peace for generations to come."
Bush again called on Shi'ite, Sunni, and Kurdish political leaders to come together to form a government, more than three months after parliamentary elections.