Karbala police chief Raid Jawdat said more than 40,000 Iraqi police and soldiers have been deployed in and around the city to avert violent attacks, along with a 10,000-strong rapid deployment force.
  
Vast flocks of Shiite pilgrims from across Iraq are on their way to Karbala, mostly on foot, to participate in Arbaeen that marks the 40th day after Ashura, when the slaying of revered seventh century Imam Hussein is marked.
  
At least 48 people were killed and 68 wounded on Sunday when a suicide bomber detonated a vest stuffed with explosives in Iskandiriyah, a town just south of Baghdad, at a rest stop used by travelling pilgrims.
  
About 60,000 Shiite faithful from abroad -- mainly from Iran but also from places like the Gulf Arab states, Tanzania and Pakistan -- are also expected at the religious event.
  
"The security is vast and organised at the highest level to accommodate the enormous number of pilgrims that will meet in Karbala," minister of state for national security Shirwan al-Waili told reporters on Monday.
  
Karbala provincial governor Akhil al-Khazali said more than five million people are expected in the city for the ceremony.
  
To prevent attacks Karbala has been transformed into a fortified citadel, surrounded by several circles of security.
  
Helicopters will fly over the crowds searching for danger, and some 750 sharpshooters will be set up in key locations for protection, security officials said.
  
Vehicles have been banned from the city, and the pilgrims entering the area must go through checkpoints to be patted down by security personnel and scanned by metal detectors.
  
The pilgrims, many of them wearing black garments, beat their chest, whip their backs and chant prayers to the beat of drums as they head to the mausoleum where Imam Hussein is buried.
  
"The security plan is going well," Waili said. "Only Iraqi forces are participating at this time, and for the moment we have not asked American forces to help us."
  
Iraqi authorities are preparing to prevent possible attacks from several different armed groups.
  
The American embassy in Baghdad and the US-led military blamed Al-Qaeda in Iraq, a Sunni group, for Sunday's attack. They did not say how they reached this conclusion.
  
"This indiscriminate violence further reflects the nature of this enemy who will target even those practicing their religion in an effort to reignite sectarian strife in Iraq," a US statement read.
  
Karbala police are especially worried about members of a millenarian radical Shiite religious group they clashed with during the Ashura celebrations in January.
  
Iraqi security forces recently seized a vast weapons cache near Karbala, without being able to determine who was hiding it. They have also found hand grenades, and even explosives hidden in flag staffs, in vehicle checks of people heading to Karbala.
  
Police said they have arrested the top leaders of the millenarian group "as well as 40 other wanted men, among them Arab foreigners," said Jawdat.
  
Karbala has been in the past the site of deadly attacks.
  
A March 2004 bombing during the Ashura festivity killed nearly 180. The attack was blamed on Sunni extremists.
  
In January 2007, a well-armed messianic group known as the Soldiers of Heaven engaged in a fierce day-long gunbattle with security forces just outside Karbala.
  
Iraqi forces said 200 group members died in the battle, in which US planes and helicopters were called out for support.
  
Last April, car bombs exploded in two separate incidents near the Karbala mausoleums, killing 42 in the first attack and 71 in the second. Hundreds were wounded in the attacks.
  
And in August, clashes between militia members allegedly belonging to the Mahdi Army, led by firebrand cleric Moqtada al-Sadr, and rival militias resulted in more than 50 killed and 300 wounded.
  
Under Saddam Hussein, Shiite processions to the holy cities were greatly limited.