Reports say at least 25,000 Iraqi security forces were deployed across Karbala, some 100 kilometers south of Baghdad, where about 2 million Shi'a have gathered for the end of the 10-day festival.
The Imam Husayn and Imam Abbas shrines are the focus of the ceremonies marking the killing of Husayn, grandson of the Prophet Muhammad, in 680.
The pilgrims included men wearing white robes who march through the streets striking their heads with swords to underscore their grief at the killing. Others beat their chests to the sound of drums and religious chants.
Similar rituals were seen in other cities across Iraq.
"These Ashura processions are a genuine expression of our love and loyalty to [Imam Hussein] who rebelled for the sake of truth, freedom, and dignity," a Shi'ite worshipper in the northern city of Kirkuk told RFE/RL's Radio Free Iraq. "[Imam Husayn], peace be upon him, restored the dignity and might of the Islamic Community. We too, in these marches, renew our pledge. Through these processions, we vow to [Imam Husayn], peace be upon him, that we will follow the path of dignity and freedom."
At least one bomb killed two people participating in the commemorations in Kirkuk.
Renewed clashes were also reported on January 19 in the southern city of Al-Nasiriyah between police and militants.
Police said at least 50 people were killed in clashes on January 18 in both Al-Nasiriyah and Al-Basrah. The violence broke out after suspected followers of a messianic Shi’ite group called the Army of Heaven attacked other Shi'ite worshippers and security forces.
During the Ashura festival last year, more than 260 members of the Army of Heaven were killed in clashes with U.S.-backed Iraqi security forces after officials purportedly uncovered a plot by the fringe group to storm the city of Al-Najaf and kill mainstream Shi'ite religious leaders.
The Army of Heaven followers are said to believe that the former leader, Dia Abd al-Zahra, who was killed in the fighting, was the Mahdi, a 9th-century messiah.
Across Shi'ite and mixed areas of Iraq, security forces have been on alert throughout the Ashura period.
In Diyala, Governorate Council Chairman Ibrahim Bajilan told RFE/RL's Radio Free Iraq on January 18 that a car ban had been imposed after suspected Al-Qaeda suicide bombers, among them a woman, attacked two Shi'ite mosques.
"After the districts of Ba'qubah and Al-Miqdadiyah were totally cleansed of all pockets of Al-Qaeda, they have been left with nothing but such attacks with explosive belts and car bombs," Bajilan said. "Thank god, the car bombs were eliminated. Explosive belts are now [carried and detonated] by women. As we are prepared, there is a car ban in all district towns today [January 18] and tomorrow [January 19]."
Meanwhile in Afghanistan, President Hamid Karzai called on Muslims to unite against "tyrants" carrying out a wave of suicide attacks in the region under the name of Islam. He was speaking to hundreds of people gathered at a Kabul mosque on January 19 for Ashura ceremonies.
"The lesson of the martyrdom of Imam Husayn and the shedding of his holy blood for us -- his followers -- is that we stand against oppression till the doom day," Karzai said. "And the Islamic world faces a major tyranny today."
Hundreds of people, mostly civilians, were killed in Afghanistan last year in dozens of suicide attacks.