Iraq's holy shrine city of Karbala was gearing up for the climax of annual Shiite mourning rituals on Wednesday, with millions of pilgrims thronging the city amid tightened security.

Officials said some 15 million devotees will have passed through Karbala by Saturday, when Arbaeen commemorations conclude, braving the threat of deadly attacks against the Shiite community that have killed dozens in the past week.

Arbaeen marks 40 days after the Ashura anniversary commemorating the slaying of Imam Hussein, one of Shiite Islam's most revered figures, by the armies of the caliph Yazid in 680 AD.

In recent days, throngs of mourners have overflowed Hussein's shrine in Karbala, demonstrating their ritual guilt and remorse for not defending him by beating their heads and chests in rituals of self-flagellation.

Sad songs blared from loudspeakers throughout the city and black flags fluttered alongside pictures of Hussein and his half-brother Imam Abbas, both of whom are buried in the city.

Karbala provincial governor Amal al-Din al-Har said he expected that some 15 million pilgrims will have visited the shrine city in the two weeks running to Arbaeen's climax on Saturday.

Included in that figure, he said, were around 200,000 devotees from outside the country.

The massive figure was overwhelming Karbala's infrastructure, the governor told AFP, warning that "there are not enough vehicles, and there are not enough roads to transport all the pilgrims."

Some 35,000 police officers and soldiers were handling security around the city, 110 kilometres (70 miles) south of Baghdad, as Shiite pilgrims carried out their traditional walk to Karbala.

Security forces have so far arrested 60 insurgents they said were planning attacks during the pilgrimage, and defused 14 roadside bombs, according to Lieutenant General Othman al-Ghanimi, who commands forces in five provinces across central Iraq.

While Karbala itself has not suffered any attacks during Arbaeen rituals, bomb attacks nationwide have targeted Iraq's majority Shiite community, with the deadliest assaults falling on January 5, when 70 people were killed in bomb blasts in Baghdad and south Iraq.

The battle near Karbala is at the heart of the historical division with Islam's Sunni and Shiite sects, a split that fuelled sectarian violence between Iraq's majority Shiite and smaller Sunni population since the 2003 US-led invasion.

Shiite pilgrims visiting Karbala are regularly targeted in attacks by Sunni extremists.

This year marks the first time Iraqi troops are solely charged with security for Arbaeen since the US-led invasion of 2003. American troops, who previously helped with surveillance and reconnaissance, completed their withdrawal from Iraq last month.

Now-executed dictator Saddam Hussein's Sunni-dominated regime barred the vast majority of Ashura and Arbaeen commemorations throughout his rule until his overthrow.

Shiites make up around 15 percent of Muslims worldwide. They represent the majority populations in Iraq, Iran and Bahrain and form significant communities in Afghanistan, Lebanon, Pakistan, India and Saudi Arabia.