Hundreds of thousands of Shia Muslims have gathered in the Iraqi city of Karbala for marking the end of the religious eve of Ashura.
The final processions will take place under tight security with more than 20,000 police deployed and marksmen placed on buildings.
Some pilgrims will march in the city, flagellating themselves with chains.
Iran, which has the world's biggest Shia population, is monitoring its own events for signs of opposition unrest.
The event mourns the 7th Century killing in Karbala of Imam Hussein (R.A), a grandson of the last prophet Hazrat Muhammad Mustafa (Peace Be Upon Him).
In previous years, Iraq has seen attacks on pilgrims attending the ceremonies by militants trying to instigate violence between Shia Iraqis and the Sunni minority.
Over the past week, police reported a number of bombings directed at Shia pilgrims.
Two pilgrims were killed on Saturday by a roadside bomb in the Iraqi capital Baghdad.
The Iraqi authorities say thousands of members of the security forces have formed cordons around Karbala.
Dogs trained to detect explosives are being used at checkpoints and helicopters are providing extra surveillance.
Checks are being made for swine flu among some 60,000 foreign pilgrims expected to attend the ceremonies, provincial governor Amal Adin al-Her told media.
Black flags of mourning have appeared across the city and most of the pilgrims are dressed in black.
"I came to show my belief in Imam Hussein," said Mohammed Abdul Hussein, 40, who travelled from the neighbouring Iraqi province of Babil.
"This event has nothing to do with personal motivations or politics."
Worshippers are also set to gather in other countries with large Shia communities such as Pakistan and Bahrain.
