Millions of Shi'ite Muslims from around the world are making their way this week to their sect's holy shrines in the Iraqi city of Karbala in an annual holy festival known as Arbaeen or Ziara.
The shrines commemorate two revered Shi'ite imams -- Hussein, the grandson of the Prophet Muhammad, and his half-brother Abbas -- and the event, which draws 10 million to 20 million pilgrims each year, will reach its peak on October 30.
By comparison, the hajj, a pilgrimage to Saudi Arabia's Mecca that all able-bodied Muslims are required to make once in their lifetime, saw only 2.4 million pilgrims this year.
Pilgrims stream toward Karbala on foot from Iraq, Iran, and places farther afield, resting along the way in tents. Some pilgrims come from as far away as Azerbaijan, Georgia, and Pakistan, though not all make the journey on foot.
Along the roads, stalls set up by charities, mosques, and devotional groups see to it that no traveler goes hungry. Cooks prepare vast amounts of stewed lamb, grilled fish, fresh bread, and rice for the pilgrims, refusing payment for the meals.
The pilgrimage marks the 40th day of mourning of the anniversary of Hussein's 7th-century death 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 and his followers were killed in the battle, cementing the schism between Sunni and Shi'ite Islam.
Sunnis today outnumber Shi'a by a wide margin among the world's estimated 1.5 billion Muslims, and Shi'ite rituals are less well-known. While the hajj is considered one of the five pillars of Islam and is an obligation for all Muslims, Sunni and Shi'ite, the Ziara is voluntary and holds little significance in Sunni tradition.
The march was forbidden for many years under former Iraqi dictator Saddam Hussein, who fought a devastating war with Iran in the 1980s. Restrictions were only lifted after his ouster in 2003.
With the formation of an Iraqi state where the post of prime minister is held by a Shi'a, the march quickly became one of the most popular pilgrimages in the world.
This year's pilgrimage is the first since Iraq's government declared victory over the Islamic State group in January, but the threat of insurgent attacks still lingers. The militant Sunni group targeted Shi'a in past attacks during the pilgrimage, as well as in high-profile attacks in the last year in Iran, Iraq, and elsewhere.
Thousands of Iraqi soldiers and special police forces have been deployed to protect the pilgrims along their travel routes.
Iranian religious affairs official Hussein Zulfighari was quoted by Iran's Fars news agency as saying that 1.7 million Iranians have already crossed into Iraq for the pilgrimage, including 50,000 Afghan Shi'a living in Iran.
Tight regulation of the hajj by Saudi authorities has driven up costs for pilgrims and deprived it of some of the spontaneity seen in the Ziara. For many Muslims who cannot afford to go on the hajj or cannot get a Saudi visa, the Ziara has become an alternative.