More than one million Shiite Muslims gathered at shrines and mosques across Iraq on Tuesday for the Ashoura religious ritual with Iraqi security forces on alert for any repeat of the attacks that have inflicted mass casualties during past pilgrimages.
The presence in the country of ultra-hardline Islamic state militants who swept through the north earlier this year raises the possibility of wider bloodshed this time as crowds swell.
Islamic State, seen as more ruthless than its predecessor in Iraq, Al Qaeda, believes Shi'ites are infidels who deserve to be killed and the group has claimed responsibility for numerous suicide bombings against members of the majority sect.
Security for the event has been tight since suspected Al Qaeda suicide bombers and mortar attacks killed 171 people during Ashoura, an event that defines Shi'ism and its rift with Sunni Islam, in Kerbala and Baghdad in 2004.
Shi'ites are commemorating the slaying of Prophet Mohammad's grandson Hussein at the battle of Kerbala in AD 680.
In the holy city of Kerbala, hundreds of thousands of pilgrims gathered outside the Shrine of Imam Hussein chanting: "Hussein, Hussein, Hussein."
During the ritual, Shi'ites beat their heads and chests and gash their heads with swords to show their grief and echo the suffering of Imam Hussein.
In the past, suicide bombers posing as pilgrims have infiltrated large crowds, and militants have fired mortar rounds at the gathering from the outskirts of Kerbala.
History of Oppression
Under Saddam Hussein's secular rule, such displays were banned in Iraq, which was ruled mostly by Sunnis in his Baath Party.
Since the dictator was toppled in 2003, Shi'ites have dominated Iraqi governments but openly practising their faith at large gatherings puts the majority sect at risk of suicide bombing attacks by hardline Sunni groups.
Islamic State's attacks on Shi'ites have helped return violence to the alarming levels of 2006-2007, the peak of a sectarian civil war.
After taking office three months ago, Shi'ite Prime Minister Haider al-Abadi promised to heal sectarian divisions in order to unite the country against Islamic State, which has declared a caliphate in parts of Iraq and Syria it controls.
But there have been no tangible signs that he is taking on Iranian-backed Shiite militias, which seem to act with impunity.
The Sunni minority, who were marginalised by Abadi's predecessor Nuri al-Maliki, complain that the militias kidnap, torture and kill at will. The militias say they only go after Islamic State militants.
Last week in western Anbar province, Islamic State killed more than 300 members of the Sunni Albu Nimr tribe, which had defied them for weeks, and dumped the bodies in mass graves or on roadsides.
Sheikh Naeem al-Ga'oud, one of the tribe's leaders, and security officials told Reuters 25 more tribesmen were shot at close range on Monday night and dumped in a well in Anbar.
During the emotional ritual in Kerbala, Shi'ites were defiant, despite the new dangers posed by Islamic State.
"Islamic State can’t stop us from coming with their violence," said pilgrim Ali Ajaj, 65.
His wife, Um Mohammed, recalled how Saddam Hussein's agents killed two of their sons, a tragedy that made her more determined to practice her faith.
"Islamic State car bombs and explosions will not stop me from coming."
Under strict security measures on Tuesday, cars were not allowed to enter Kerbala for fear of car bomb attacks. Instead, pilgrims boarded buses organised by authorities.