With just hours to go until the Arbaeen commemoration reaches its climax tomorrow, a sea of black-clad devotees filled the streets, waving flags, beating their chests and chanting.
A mortar attack near the city earlier in the day killed one person and wounded four, highlighting the security concerns surrounding the annual event.
Nine rounds hit an area called Souk al-Basra, around seven kilometres (four miles) from the city centre, a police colonel said, adding that the casualties were local residents and not pilgrims.
Hussein is one of the most revered figures for Shiites, who are the largest community in Iraq and the overwhelming majority in neighbouring Iran.
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The jihadist organisation -- led by Sunni extremists -- considers Shiites to be heretics and has made targeting the community one of its main objectives.
Brigadier General Qais Khalaf Rahim, head of operations command for the Karbala area, said "the number of visitors is far greater than last year's".
"We had to open more routes to Karbala and double the number of cars, buses and trucks transporting pilgrims to the city," he told reporters.
Grand Ayatollah Ali al-Sistani, the most revered Shiite cleric in Iraq, urged the authorities in his Friday sermon to be better prepared for such a huge influx of visitors.
Speaking through one of his representatives, Sheikh Abdelmahdi al-Karbalai, he said a comprehensive and scientifically sound plan should be devised to move "beyond chaos and improvisation".