Karbala, Reuters—More than one million Shi’ite Muslims gathered at shrines and mosques across Iraq on Tuesday for the Ashura 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 of Iraq and Syria (ISIS) militants who swept through the north earlier this year raises the possibility of wider bloodshed this time as crowds swell.

ISIS, 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 Ashura, an event that defines Shi’ism and its rift with Sunni Islam, in Karbala and Baghdad in 2004.

Shi’ites are commemorating the slaying of Prophet Mohammad’s grandson Hussein at the battle of Karbala in AD 680.

In the holy city of Karbala, 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 Karbala.

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 practicing their faith at large gatherings puts the majority sect at risk of suicide bombing attacks by hardline Sunni groups.

ISIS’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 ISIS, which has declared a caliphate in the parts of Iraq and Syria it controls.

But there have been no tangible signs that he is taking on Iranian-backed Shi’ite militias, which seem to act with impunity.

The Sunni minority, who were marginalized by Abadi’s predecessor Nuri Al-Maliki, complain that the militias kidnap, torture and kill at will. The militias say they only go after ISIS militants.

Last week in western Anbar province, ISIS 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 Karbala, Shi’ites were defiant, despite the new dangers posed by ISIS.

“Islamic State [ISIS] 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 Karbala for fear of car bomb attacks. Instead, pilgrims boarded buses organised by authorities.