Imam Hussein's killing in the Battle of Karbala in 680 AD was part of a fierce dispute over who should succeed the Prophet, which eventually developed into a bitter schism between the Sunni and Shiite branches of Islam.

Under tight security, around 14 million worshippers crowded into the golden-domed mausoleum where the Prophet Mohammed's grandson is buried, Karbala's religious authorities said, beating their chests in unison against a background of religious music.

of Imam Hussein by the forces of the Caliph Yazid -- a formative event in Shiite Islam.

Arbaeen is one of the biggest religious festivals on earth and marks the end of the 40-day mourning period for the 7th-century killing

An estimated 14 million Shiite pilgrims thronged Iraq's holy city of Karbala on Friday to mark the annual Arbaeen commemoration, as Baghdad looks to wipe out the Islamic State group that has targeted their branch of Islam.

Shiite pilgrims gather in front of the Imam Hussein shrine in the southern Iraqi city of Karbala for the Arbaeen religious festival amid high security (photo by: Mohammed SAWAF/AFP)

Tens of thousands of security personnel and Shiite militiamen were deployed, as in past years, around the perimeters of the sanctuary as well as on all roads leading to Karbala, about 80 kilometres (50 miles) southwest of Baghdad.

The expulsion of IS from Iraqi cities has added an extra dimension to this year's pilgrimage, as the Sunni extremist group has repeatedly targeted Shiites. Last year, a suicide bombing killed at least 70 mainly Iranian worshippers returning from the commemoration.

The jihadists have seen their self-styled "caliphate" disintegrate on the battlefield, and are currently fighting to hold a last pocket of territory in Iraq on the Syrian border.

Iraqi Prime Minister Haider al-Abadi said the security operation to protect the millions of pilgrims was a "success" and "another victory" for Iraqi forces who have routed IS from most of the country.

'Finished with IS'

Believers from across the Middle East and beyond made the pilgrimage to Karbala.

A picture taken on November 10, 2017 shows Shiite pilgrims praying at the Immam Hussein shrine in the southern Iraqi city of Karbala during the Arbaeen religious festival (photo by: HAIDAR MOHAMMED ALI/AFP)

Shiites make up some 60 percent of Iraq's population.

Ahead of the peak of the commemorations local governor Aqil Tourihi told AFP that over 10 million Iraqis had arrived.

More than two million Iranian pilgrims also crossed the

border into Iraq for Arbaeen, an Iranian official said.

The overall attendance of around 14 million appeared down on the roughly 17 to 20 million people estimated to have attended Arbaeen in 2016.

"There are pilgrims coming from Arab countries, from Turkey, from the US, the Gulf, from everywhere in the world, and this is the proof that Muslims all over the world are united," said Shia al-Mussawi, a pilgrim visiting from Iraq's Misan province.

Helicopters flew overhead to ensure security in the city, located some 300 kilometres (185 miles) southeast from where Iraqi forces are battling to snuff out IS.

Along hundreds of kilometres of road leading to Karbala, thousands of volunteers greeted the pilgrims, most of whom walked to the Imam Hussein shrine, to offer food, drink and a variety of services.

A volunteer massages a Shiite pilgrim on the road to the Imam Hussin shrine (photo by: HAIDAR MOHAMMED ALI/AFP)

Behrouz Mahdavi, a pilgrim from Iran, brought with him a six-metre-long tent in which he invited pilgrims aching from the long walk to rest while he massaged their feet.

Calling themselves "servants of Imam Hussein" these volunteers took pride in preparing tea for the pilgrims, handing out water bottles, some dates or even home-cooked meals for the hungry.

Tahsin Jundi al-Hassan and Ahmed Rahim set up a "taxi service" offering pilgrims a free ride on a wooden cart for the last few kilometres to the shrine.

"The conditions are perfect, above all in terms of catering for pilgrims and security," Kuwaiti pilgrim Fadel Yaqoub told AFP.

Bandar al-Hamami brushed off the concerns about security after walking some 350 kilometres with his family from their home in the southern city of Nasiriyah.

"Thanks be to God, we are finished with IS," he said.