The pilgrims are prime targets for the IS jihadists, who have carried out a series of mass executions in recent days, killing scores of members of a tribe in Iraq's western Anbar province.
Hundreds of thousands of pilgrims from Iraq and abroad are heading to Karbala for tomorrow's peak of Ashura, which marks the death of Imam Hussein, one of the most revered figures in Shiite Islam.
At least 19 people were killed in bomb blasts targeting Shiites in Baghdad yesterday and Iraqi security forces were on alert for further attacks.
Authorities have deployed thousands of security personnel and allied militiamen to protect the pilgrims, in what will be a major test for the new government headed by Prime Minister Haidar al-Abadi.
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"The security plan is fully in effect and the security forces are on a state of high alert," an Iraqi police colonel told AFP, speaking on condition of anonymity.
Police were deployed throughout Shiite neighbourhoods of Baghdad and security forces are guarding the 100-kilometre route from the capital to Karbala.
At the entrance to the city, police were using X-ray trucks to scan vehicles and sniffer dogs were checking arrivals. Some 1,500 policewomen were also deployed to check female pilgrims.
The Sunni extremist IS group has declared a "caliphate" in parts of Iraq and neighbouring Syria under its control, imposing its harsh interpretation of Islamic law and committing widespread atrocities.