A curfew will be slapped on Baghdad and 10 Iraqi provinces on Thursday for the three-day Shiite Muslim festival of Ashura, state television reported on Wednesday.
All traffic will be banned from Thursday night in nine southern provinces as weel as in Baghdad and the Diyala province in the centre-north of the country where many Shiites live, the channel quoted an interior ministry statement as saying.
Up to a million pilgrims are expected to descend on Karbala in time for the climax of the annual rituals on Saturday.
Many travel on foot and in past years have been exposed to attacks by Sunni insurgents.
Police have said tens of thousands of Iraqi troops and police would be on duty in Karbala and nearby Najaf for Ashura, which marks Shiite Islam's holiest days.
Some 12,000 Iraqi soldiers and police have been deployed along with 3,000 members of a police rapid response unit in Karbala, according to city police.
Last August a pilgrimage in Karbala became a bloodbath when police and gunmen of the Mahdi Army militia of radical cleric Moqtada al-Sadr clashed at two holy shrines in the city centre.
Sadr suspended the activities of his militia two days after the clashes, which killed 52 people and ended the pilgrimage abruptly.
Police are also on alert in Najaf, site of the shrine of Imam Ali and headquarters of revered Shiite cleric Grand Ayatollah Ali al-Sistani, which is one of the main stopping points on the way to Karbala..
Some 4,000 officers are patrolling the 50-kilometre (30-mile) route between Najaf and Karbala.
Checkpoints have been set up along all routes to Karbala and the security forces are using special equipment to detect explosives, police said.
In the past suicide bombers have mingled among crowds of pilgrims before detonating their explosive vests, causing carnage.
Ashura, which means the tenth in Arabic, falls on the 10th day of the Muslim month of Muharrem.
The climax of Ashura, which commemorates the killing in Karbala of Imam Hussein by armies of the Sunni caliph Yazid in 680, falls on January 19.
Tradition holds that Hussein, a grandson of the Prophet Mohammed (pbuh), was decapitated and his body mutilated by Yazid's armies.
To express remorse and guilt for not saving Hussein, Shiite volunteers flay themselves with chains or slice their scalps during processions to the Karbala shrines.