Millions of black-clad Shiite Muslims, crying and beating their chests in mourning, streamed through a shrine in the Iraqi city of Karbala Monday under heavy protection from the security forces.

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.

Some pilgrims came from as far afield as Basra, Iraq's main southern port city some 500 kilometres away by road.

"I came walking from Basra with my wife and three sons... This is the third time," said Jaber Kadhem Khalif.

"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.

- Support for Mosul -

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.

"We ask God to support us against Daesh (IS) members, to help us liberate Mosul and urge our politicians to remember the people who have sacrificed so much," she 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 last day of Arbaeen was dominated by a controversy in Iraq over the publication by Saudi newspaper Asharq al-Awsat of an article accusing Iranian pilgrims of sexually harassing women during the commemoration.

- Newspaper controversy -

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.

"I want to respond to what was published in Asharq al-Awsat. They want to slander the pilgrims and this event," said Magdi al-Muslim, a Shiite who is himself from Saudi Arabia.

"Our message to them is that we have Imam Hussein and we won't leave him," he said.

The London-based paper said Monday that it had sacked its Baghdad correspondent over the false new story.

Baghdad and much of the country south of the capital come to a standstill in the days preceding Arbaeen, as several major motorways are reserved for pilgrims on foot on one side and authorised vehicles on the other.

Since the previous Arbaeen in December 2015, IS's "caliphate" has been unravelling and the jihadists are expected to increasingly turn to spectacular one-off attacks on symbolic targets.

There were some attacks during Arbaeen this year, but with far fewer victims than in previous years.

Karbala, about 80 kilometres (50 miles) southwest of Baghdad, faces the desert of Anbar, a vast province which was until recently an IS bastion and where jihadists still carry out frequent attacks.