Millions of pilgrims have made their way on foot to the Iraqi city of Karbala for the Shia pilgrimage of Arbaeen, regarded as the largest annual public gathering in the world.

The commemoration marks the 40th day following the death of a Shia saint in the 7th century, and includes more than two million Iranians and other Shias from abroad.

Militias patrolled roads leading into the city and escorted Iranian pilgrims from the border, boosting security for the processions which have previously been the target of bombings by Sunni militant groups.

Iraq pilgrimage
The gathering is the biggest public gathering in the world, and draws more pilgrims than the annual Hajj (AP)

This year’s Arbaeen ceremonies take place amid widespread anger in Iraq’s Shia south over the government’s heavy crackdown on protests that erupted earlier this month against unemployment, corruption and government mismanagement.

The demonstrations raged across Iraq for seven days and most prominent among the protesters were young Shias, unleashing their frustration over the Shia-led government in Baghdad.

The security crackdown, which killed more than 100 people and wounded thousands more, put down the protests last week, but a new round of demonstrations has been called for October 25.

The political turmoil surfaced in the Arbaeen ceremonies.

Followers of populist cleric Muqtada al-Sadr marched toward Karbala chanting: “No to America, no to Israel, no to corruption” and “Baghdad is free, corruption must go!”

Shia pilgrims
Iranian Shia pilgrims perform their rituals around the holy shrine of Imam Hussein (AP)

In a message marking the day, Iraqi prime minister Adel Abdul-Mahdi vowed to “confront with strength and determination all forms of corruption and achieve justice”.

The 77-year-old premier, who took office last year, has promised to address protesters’ demands but has also told them there is no “magic solution” for Iraq’s accumulated problems, including high unemployment, corruption, dilapidated public services and poor security.

Pilgrims streamed towards Karbala on foot from the cities of Najaf, 45 miles away, Baghdad, 55 miles to the north, and other places farther afield, resting along the way in tents.

The pilgrimage, known in Arabic as the Ziara, marks the anniversary of the 40th day of mourning following the 7th century death of the Prophet Muhammad’s grandson Hussein at the hands of the Muslim Umayyad forces in the Battle of Karbala, during the tumultuous first century of Islam’s history.

Hussein was seen by his followers as the rightful heir of the prophet’s legacy. When he refused to pledge allegiance to the Umayyad caliphate, he was killed in the battle, cementing the schism between Sunni and Shia Islam. Hussein’s half-brother Abbas was also killed in the battle.

Sunnis outnumber Shias by a wide margin among the world’s estimated 1.5 billion Muslims, and Shia rituals are far less known.

Iraq pilgrimage
Pilgrims have travelled from far and wide (AP)

But Arbaeen – Arabic for the number 40 – draws far more pilgrims than the Hajj in Saudi Arabia, a pilgrimage required once in a lifetime of every Muslim who can afford it and is physically able to make it.

The Hajj is considered one of the five pillars of Islam and an obligation for all Muslims – Sunni and Shia. The Ziara is voluntary and holds little significance in Sunni tradition.

In recent years, the Iraqi government said Karbala received 10-20 million visitors during the event. Saudi authorities regulate the hajj tradition tightly, driving up costs for pilgrims and depriving it of some of the spontaneity seen in the Ziara.

For many Muslims who cannot afford to go on the Hajj or cannot obtain the Saudi visa, the Ziara is a satisfying alternative.

In neighbouring Shia-majority Iran, Arbaeen is a national holiday. Thousands in Tehran marched toward a nearby town south of the capital to mourn at the shrine of Shia saint Abdul Azim.

Iran’s deputy interior ministry, Hossein Zolfaghari, said that more than 3.4 million Iranians travelled to Iraq and two million of them have returned.