Millions of Shiite Muslim pilgrims massed at the golden-domed mausoleums of the holy Iraqi city of Karbala on Wednesday, commemorating Arbaeen, one of the largest religious gatherings in the world.

The event, organised under strict security, brought together some 22 million pilgrims this year, according to official figures.

Iran hit a new participation record with four million visitors, a top security official told the Iranian news agency IRNA, up from three million last year.

Marked by Shiites, who constitute the majority in Iraq and Iran, Arbaeen ("Forty" in Arabic) marks the 40th day of mourning for the martyr Imam Hussein, grandson of the Prophet Mohammed and a founding figure of Shiite Islam.

Karbala, where Hussein and his brother Abbas are buried in two enormous mausoleums facing each other, is an epicentre of Shiite Islam.

Pilgrims freely expressed their suffering, weeping and wailing in memory of Hussein, who was killed in 680 during a battle in Karbala with the Umayyad caliph Yazid.

Pilgrims dressed in black, some sporting headbands bearing religious messages, moved forward shoulder-to-shoulder to enter the mausoleums and pray.

As they massed on the esplanade between the mausoleums, mist machines struggled to provide cooling in 41-degree Celsius heat (106 Fahrenheit).

Accompanied by religious chants and prayers, processions of the faithful holding up black banners with Hussein's image moved around the two mausoleums and the esplanade.

The pilgrimage reached its climax on Wednesday, but the faithful had already been converging for several days on Karbala.

Iraqi Prime Minister Mohamed Shia al-Sudani visited Karbala on Tuesday evening, where he said: "The state has mobilised all of its resources to serve its citizens", according to a statement from his office.

He also hailed the volunteers from across Iraq's provinces who preserve the tradition of setting up and financing "mawakeb", stands that serve free drinks and food along the pilgrim routes.