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.
The jihadists are reported to have slaughtered dozens of members of the Sunni Albu Nimr tribe, which took up arms against them in Anbar.
Today, tribal leader Naim al-Kuoud al-Nimrawi told AFP that IS "executed 36 people, including four women and three children" yesterday alone.
A police officer and an official gave figures of more than 200 to 258 people killed, while Iraq's human rights ministry put the toll at 322 and a tribal leader said 381 were executed.
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The mass killings appear aimed at discouraging resistance from powerful local tribes in Anbar, where IS overran large areas in June as pro-government forces suffered a string of setbacks.
Hundreds of thousands of pilgrims are expected in Karbala for tomorrow's peak of Ashura marking the death of Imam Hussein, one of Shiite Islam's most revered figures.
Karbala deputy governor Jassem al-Fatlawi told AFP "hundreds of thousands of Iraqi pilgrims" and 65,000 others from 20 different countries have thronged Karbala.
Pilgrims have been targeted during Ashura before, but this year they face even greater danger after the IS lightning offensive in June.
Like other Sunni extremists, IS considers Shiites heretics.
Authorities have deployed thousands of security personnel and allied militiamen to protect the pilgrims, in a major test for the new government of Prime Minister Haidar al-Abadi.
Police were deployed throughout Shiite districts of Baghdad and security forces are guarding the 100-kilometre route from the capital to Karbala.
More than 26,000 members of security forces were deployed in Karbala itself, with choppers providing air support, army Staff Lieutenant General Othman al-Ghanimi told reporters.
Police used X-ray trucks to scan vehicles as sniffer dogs monitored arrivals and some 1,500 policewomen checked female pilgrims.