Strict security measures have been imposed in Iraq to prevent insurgent attacks during the major Shia religious festival of Ashura.
Shia festivals have often been the target of insurgent attacks
Roads have been closed to vehicles and checkpoints set up in Baghdad and the holy city of Karbala in central Iraq.
In 2004, 170 people were killed in co-ordinated attacks during Ashura.
Despite the restrictions, millions of pilgrims have been taking part in ceremonies commemorating the martyrdom of the Imam Hussein.
Falling on the 10th of Muharram, the first month of the Islamic lunar calendar, Shia Muslims mark the death of the Prophet Muhammad's grandson at Karbala with mourning rituals and plays re-enacting the martyrdom.
Men and women dress in black and parade through the streets slapping their chests and chanting.
Some Shia orders flagellate themselves with chains and blades to emulate Hussein's suffering - though this is frowned upon by others.
The US ambassador in Iraq, Zalmay Khalilzad, called on Iraqis to unite ahead of the festival.
SHIA FESTIVAL: ASHURA
Annual Shia festival commemorating martyrdom of Imam Hussein
Hussein, grandson of Prophet Muhammad, killed at Karbala by army of Caliph Yazid in 680
The killing 19 years earlier of Ali, Hussein's father, gave rise to the central schism in Islam between Sunni and Shia
"All Iraqis, regardless of their religion or sect, should unite after decades of oppression in the common purpose of national renewal," he said.
Approximately 8,000 security personnel, including 2,000 undercover officers, have been deployed to protect the pilgrims in Karbala.
Checkpoints have also been set up to search pilgrims before they can enter the Imam Hussein shrine, and hotels have been searched for explosives.
In Baghdad, police closed main roads in the mainly Shia district of Kadhimiya.
Almost 1,000 people died during the last major Shia festival in August when a stampede was triggered on a bridge by rumours that suicide bombers were preparing to blow themselves up.
The leader of one of Iraq's two main Shia parties, Abd al-Aziz al-Hakim, has called for Iraq's "faithful security forces" to combat such attacks, but with "more consideration for human rights" and respect for Iraq's new constitution.
Mr Hakim said Iraqis needed to work together to "rescue our country" and enable "multinational forces to return to their countries."
Shias regularly depict the Imam Hussein
"That is what we want for Iraq and Iraqis."
Mr Hakim also criticised the cartoons satirising the Prophet Muhammad that were published in European newspapers this month.
But he condemned violent attacks on foreign embassies throughout the Muslim world.
"We value and appreciate peaceful Islamic protests held in the different areas around the world," he said.
"But we are against the idea of attacking embassies and other official sites."
On Wednesday, the convoy of Higher Education Minister Sami al-Muzaffar was attacked in Baghdad.
Mr Muzaffar escaped unhurt when a bomb exploded near his car in the central Karrada district, but three of his bodyguards were lightly wounded.
The attack was claimed by the Mujahidin Shura Council, an umbrella body set up on 15 January by al-Qaeda in Iraq and five other militant groups to co-ordinate the fight against multinational forces, and secular and Shia groups.
In addition to Abu Musab al-Zarqawi's group, the council includes Jaish al-Taifa al-Mansoura, Ansar al-Tawhid, al-Ghuraba, al-Jihad al-Islami and al-Ahwal.