Millions of pilgrims have converged on Iraq’s holy city of Karbala to commemorate Arba’een, the 40th day since the martyrdom anniversary of Imam Hussein (PBUH), the third Shia Imam.

The occasion, known as the world’s largest annual Muslim pilgrimage, comes 40 days after Ashura, the 10th day of the lunar calendar month of Muharram when Imam Hussein, the grandson of Prophet Muhammad (PBUH), was martyred in the Battle of Karbala at the hands of the tyrant of the time, Yazid I, in the seventh century

Each year, Shia and Sunni pilgrims as well as those from other faiths gather in Iraq in the weeks leading to the day of Arba’een and stream towards Karbala on foot from other Iraqi cities, especially Najaf and Basra.

Pilgrims are seen on their way to Imam Hussein (PBUH)’s Holy Shrine, in Karbala, Iraq, on October 23, 2018. (Photo by IRNA)

During the long walks, the pilgrims sleep and eat in tents erected by charity groups and mosques alongside roads.

This year's pilgrimage culminates on Tuesday. Millions of pilgrims have already gathered in Karbala to join the mourning procession around the golden-domed shrines of Imam Hussein and his half-brother Hazrat Abbas.

Pilgrims are seen on their way to Imam Hussein (PBUH)’s Holy Shrine, in Karbala, Iraq, on October 23, 2018. (Photo by IRNA)

In recent years, Karbala has been host to between 10 and 20 million visitors during the event.

At least two million pilgrims have passed border checkpoints into Iraq from neighboring Iran.

In the Iranian capital Tehran, thousands of people are taking part in a large symbolic march marking Aba’een.

Arba’een is viewed as an overwhelmingly powerful display of Shia solidarity and conveys a message of unity among Muslims against the global arrogance.

In a Twitter post on Monday, Iran’s Foreign Minister Mohammad Javad Zarif said that the annual pilgrimage bears the message that “no tyrant will ever end the struggle for justice.”

14 centuries ago a tyrant believed he had eradicated Imam Hussein's resistance. Saddam, Al-Qaida & ISIS thought they could end it. Yet today 17M from across the world—w/2M Iranians—are amassing in Karbala. Their message: No tyrant will ever end the struggle for justice. #Arbaeen pic.twitter.com/cQeyPf06KN — Javad Zarif (@JZarif) October 29, 2018

Imam Hussein is a highly-venerated figure not only among Shia Mulims but also among Sunnis, Christians, and people of other faiths.