Shia Muslims gathered at shrines and mosques across Iraq on Tuesday for the Ashoura religious ritual as Iraqi security forces were on alert for possible attacks that have inflicted mass casualties during past pilgrimages.
The presence of ultra-hardline Islamic state militants in the country who swept through the north raises the possibility of wider bloodshed this year as crowds swell into the millions.
Islamic State, seen as more ruthless than its predecessor in Iraq, al Qaeda, believes Shias 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.
Shias are commemorating the slaying of Prophet Mohammad's grandson Hussein at the battle of Kerbala in AD 680.
During the ritual, Shias beat their heads and chests and gash their heads with swords to show their grief and echo the suffering of Imam Hussein.
Under Saddam Hussein's secular rule, such displays were banned in Iraq, which was ruled mostly by Sunnis in his secular Baath Party.
Since the former dictator was toppled in 2003, Shias 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 Shias have helped return violence to the alarming levels of 2006-2007, the peak of a sectarian civil war.
Under strict security measures on Tuesday, cars were not allowed to enter Kerbala for fear of cars bomb attacks. Instead, pilgrims boarded buses at organised by authorities.
In the past, suicide bombers posing as pilgrims infiltrated large crowds and carried out suicide bombings, or fired mortar rounds at the gathering from the edge of Kerbala.