Millions flooded Iraq's shrine city Karbala on Tuesday for Ashura rituals on Shiite Islam's most important day, amid tight security after bomb attacks against pilgrims left 28 people dead.

Groups of pilgrims walked the streets beating their chests or flaying their backs with chains, ritually mourning the death of Imam Hussein, a grandson of the Prophet Mohammed, who was killed by armies of the caliph Yazid near Karbala in 680 AD.

Tradition holds that the revered imam was decapitated and his body mutilated. Hussein's body is buried in Karbala, which is about 100 kilometres (60 miles) south of Baghdad. His death was a formative event in Shiite Islam.

The processions of mourners, many dressed in black, include a person reciting poetry concerning Hussein's death over a loudspeaker. Some people also carry pictures depicting the battle in which he was killed.

The red flags representing the blood of Hussein and his half-brother Abbas, who was also killed near Karbala, that usually fly over their respective shrines in the city were changed to black, to represent mourning.

Later in the day, the attack that killed Imam Hussein was to be reenacted near the shrine where his body is buried.

"The number of visitors to Karbala to commemorate Ashura reached about three million" people, Karbala governor Amal al-Din al-Har told AFP.

Iftikhar Abbas, the head of the province's tourism committee, said the visitors included some 650,000 foreigners -- 430,000 from Europe, the United States, Iran, India, and Pakistan, and 220,000 from Syria, Egypt, Kuwait, Saudi Arabia and other Arab countries.

The city's roughly 400 hotels were full and pilgrims were renting private homes, she said.

Tuesday's rituals took place amid a heavy deployment by Iraqi security forces.

Lieutenant General Othman al-Ghanimi, the head of Al-Farat al-Awsat operations command, which covers the Karbala area, said that 28,000 police and army personnel were deployed in and around Karbala to protect pilgrims.

Iraqi helicopters are playing a role in the security plan, he added.

On Monday, at least 28 people were killed and 78 wounded in a wave of bomb attacks in central Iraq against Shiite pilgrims.

And three civilians and two police were wounded on Tuesday by two mortar shells fired at a Shiite mosque in the disputed north Iraq city of Kirkuk, police said.

The 10-day Ashura commemorations began on November 27, peaking in Karbala on Tuesday.

Shiites gather at night during the commemorations to listen to stories about Hussein's family and other companions who were killed prior to his death on the 10th day of Muharram, the first month of the Islamic Hijra calendar year.

The Ashura rituals have frequently been targeted by Sunni Arab extremists, who see them as symbolically highlighting the split between Islam's two main branches.

Iraq's Sunni minority and Shiite majority engaged in a bloody sectarian conflict beginning in 2006 that left tens of thousands of people dead.

Now-executed dictator Saddam Hussein's Sunni-dominated regime barred the vast majority of Ashura commemorations throughout his rule until his overthrow in the 2003 US-led invasion.

The deadliest Ashura attacks were in March 2004 when near-simultaneous bombings in Karbala and at a Shiite mosque in Baghdad killed more than 170 people.

Violence levels are much lower now than at their peak in 2006 and 2007, when communal bloodshed engulfed much of the country.

But attacks remain common however and a total of 187 people were killed in November, according to official figures.

Shiites make up around 15 percent of Muslims worldwide. They represent the majority populations in Iraq, Iran and Bahrain and form significant communities in Afghanistan, Lebanon, Pakistan and Saudi Arabia.