Heavy Iraqi security is being deployed in the Shia Muslim shrine city of Karbala for Ashura, which marks the slaying of the revered Imam Hussein by the armies of the Sunni caliph Yazid in 680.
The holy city has in previous years been a target for sectarian violence.
It comes with barely more than a year before the 50,000-odd remaining American soldiers in Iraq must withdraw from the country completely, under the terms of a bilateral security pact.
"Karbala will be 100 per cent protected by Iraqis, with no participation from American forces unlike in previous years," said army General Othman Al-Ghanimi, commander of Iraqi security forces in five central provinces including Karbala.
While US forces have not supplied ground forces to Karbala for Ashura in several years, they have previously provided surveillance and reconnaissance. Last year, for example, they helped provide air surveillance.
This year however, these tasks would be handled by Iraqis, Ghanimi said.
Some 28,000 soldiers and policemen would be deployed to protect the city, with a further 7,000 available if needed, he said. Among them are some 600 policewomen to search female travellers to counter women suicide attackers, who have struck Karbala before.
All pilgrims entering Karbala, which is home to the shrines of Imam Hussein and his half-brother Imam Abbas, are to be searched, with bomb-detection devices, explosives-sniffing dogs and vehicle scanners being used.
Six security perimeters have been established around the city, with a particular focus on entrances to Karbala and its old city, close to Imam Hussein's shrine.
Surveillance cameras and helicopters, piloted by Iraqi officers, will monitor pilgrims' movements throughout the city.
"The Americans are not participating in anything this year," said Karbala police spokesman Major Alaa Abbas. "They will only be called if there is a necessity."
Captain Leslie Waddle, a US military press officer, said in an email that Iraqi forces would be the "lead provider" of security during Ashura, and that US forces would remain in an "advise and assist role," but did not elaborate.
Insurgents have already targeted the 10-day Ashura rituals this year.
On Monday, a suicide bomber detonated explosives in the Diyala provincial town of Baladruz, 75 kilometres (50 miles) north of Baghdad, killing four Shia and wounding 17 others.
That attack came a day after another suicide bomber blew himself up in the middle of a Shia procession in Baquba, the capital of Diyala province, killing three people.
The ceremonies, which climax on Friday, have been targeted in previous years by insurgents.
Most notably, in March 2004, near-simultaneous bombings at a Shia mosque in Baghdad and in Karbala killed more than 170 people.
Travellers have also been hit at rest-stops between Baghdad and Karbala, a 110-kilometre (70-mile) route pilgrims often cover on foot.
As a result, security is dramatically ramped up for Ashura, with two million people expected in Karbala on Friday, including 100,000 foreigners, provincial governor Amal Al-Din Al-Har said.
While thousands of pilgrims visit Karbala and other major Shia shrines in Samarra, Najaf and Baghdad, every day -- many from Iran and other countries with large Shia populations -- the number peaks during Ashura.
The massive influx means Karbala's 320 hotels have been filled, and local families are now opening their homes to travellers, provincial tourism official Ahmed Abdul Hussein said.
"We have a complete service plan to deliver water, electricity, transportation and food to all of them," Har added.