HOLY CITY OF KARBALA, Iraq: Iraqi officials said an estimated twenty-seven millions mourning pilgrims have gathered in the holy city of Karbala to mark Arbaeen, also known as Chehlum, which commemorates the end of the 40-day mourning period after the martyrdom of Imam Hussein, the Prophet Muhammad's grandson, in 680 AD.

Arbaeen is the world's largest annual gathering, with the number of pilgrims far exceeding the two million visitors who descend on the holy city of Makkah for the Hajj.

The faithful from over 100 countries had for days been streaming through the golden-domed mausoleum of Imam Hussein (as) despite the threat of terror attacks.

Many made the pilgrimage on foot from Baghdad, a journey of 80km which can take days. Some walked from as far as Basra in southern Iraq, a journey of nearly 700km. Hundreds of thousands of pilgrims came from neighbouring Iran; a journey that takes about a month on foot.

“I arrived in Karbala after walking for 12 days,” said Mohammed Hussein Jassem, a laborer from Iraq’s deep south.

“I found a lot of services on the road,” he said, referring to the thousands of “mawakeb”, spots where tents are erected by volunteers to serve the pilgrims food and beverages.

Although Arbaeen is a Shia ritual, Sunni Muslims and people from other religions such as Christianity and Zoroastrianism also made the pilgrimage.

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

Iraqi officials also said they had adopted special security measures this year to protect pilgrims against possible terrorist attacks.