The processions of faithful walking to Imam Hussein's mausoleum from all over Iraq have been routinely targeted by bombings over the years, but this time the Arbaeen commemoration saw very few attacks.
The Islamic State group is battling tens of thousands of Iraqi forces in and around its last major stronghold of Mosul, and observers had feared it might seek to strike Baghdad or Karbala during Arbaeen.
The authorities in Karbala, which lies about 80 kilometres (50 miles) south of Baghdad, said at least 24,000 soldiers and police were securing the area.
"I came walking from Basra with my wife and three sons... This is the third time," said Jaber Kadhem Khalif.
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"We started walking 13 days ago and reached Karbala on Sunday night."
The 40-year-old said his prayers would go to the Hashed al-Shaabi paramilitary group which has tens of thousands of men deployed on the front lines to fight IS.
Umm Ali came without her husband, who is with the security forces on the front lines.
"I came with my son and two daughters. I came from Samawa to Najaf by car, then from Najaf to Karbala on foot, to make a plea for my husband's safety," the 45-year-old said.
Arbaeen, which means "40" in Arabic, is an observance that peaks on the 40th day after the anniversary of Imam Hussein's 680 AD death, but the pilgrimage takes place over several days running up to it.
Nusayyef al-Khattabi, who heads the Karbala provincial council, said he expected the total number of visitors over several days to range "between 17 and 20 million".
Among them are an estimated three million foreigners, mostly Iranians who started crossing the border days ago.
The article quoted a spokesman from the UN's health agency, which denied any such comment was ever made.
It drew condemnation from Prime Minister Haider al-Abadi and several other leading Shiite figures and bodies demanding an apology.
It kept Saudi-Iranian tension as a backdrop to the pilgrimage, after the deadly 2015 stampede during the hajj pilgrimage to Mecca prompted Tehran to angrily question Saudi custodianship of holy Muslim sites.
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