Millions of people from across the world have gathered in Iraqi holy city of Karbala to mark the anniversary of the 40th day after the martyrdom of Imam Hussein, the grandson of Prophet Muhammad.

The pilgrims, many of whom took part in the 76 kilometers march from Najaf to Karbala, are observing Arbaeen, which is known as the world’s largest annual religious gathering on earth.

Arbaeen is observed 40 days after the day of Ashura, during which the third Shia Imam was martyred at the hands of the tyrant of the time, Yazid, back in the seventh century.

Iraq has tightened security measures in and around Karbala, which is home to Imam Hussein’s holy shrine. Tens of thousands of security officers have been stationed in Karbala as well as on all roads leading to the city.

As pilgrims march toward Karbala, they are provided with food and drinks from tents which have been erect along all roads toward Karbala.

Pilgrims come from all across the world, including Iran, Azerbaijan, Turkey, Yemen, Oman, Bahrain, India, Pakistan and Lebanon.

This year, nearly 2.3 million pilgrims traveled to Iraq to mark Arbaeen.

Imam Hussein is also a respected figure among Sunnis, Christians and the people of all other faiths.