Roads have been sealed off and checkpoints added to protect the expected 20 million pilgrims from terrorist attack, Tasnim news agency reported Monday.
The pilgrimage celebrates Arbaeen, the end of 40 days of mourning on the anniversary of the martyrdom of the Prophet Muhammad's grandson, Imam Hussein.
About 1.3 million foreigners are among millions of worshipers who have already entered the city, local officials say.
Observance of Arbaeen was prohibited for 30 years during the reign of Saddam Hussein.
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