Thousands of Iraqi security personnel guarded areas in and around Karbala on Tuesday to protect hundreds of thousands of Shiite pilgrims flocking to the shrine city for annual mourning rituals.

Shiites in Iraq have come under frequent attack by Sunni extremists of the Islamic State group who regard them as heretics and who still control some territory in Anbar province, to the west of Karbala, though attacks in the city itself are rare.

Imam Hussein, the grandson of the Prophet Mohammed, is buried in Karbala, and Shiite pilgrims pack the city each year for Ashura commemorations, which mark his death in the 7th century.

"Our forces from the army and local and federal police took strict security measures culminating today to protect pilgrims in and around Karbala," police spokesman Colonel Alaa al-Ghanimi told AFP.

"Forces from the Hashed al-Shaabi have implemented security measures to control areas in the west of Karbala province," Ghanimi said, referring to an umbrella organisation for pro-government militiamen.

According to Ghanimi, some 30,000 security personnel are taking part in efforts to guard the city and its surroundings.

On Tuesday, pilgrims sang songs about the story of Imam Hussein's death, some beating their chests with their hands or their backs with chains in mourning.

Black banners were draped around the city, and pilgrims, most of them black-clad, packed the streets around the shrines of Imam Hussein and his brother Abbas.

The number of pilgrims participating in the Ashura commemorations is expected to be huge, with Karbala deputy governor Ali al-Mayali saying it was expected to reach three million.

Some 250,000 pilgrims have arrived over the past two days, Mayali said.

Ghanimi said there had been no security breaches so far, and while IS frequently targets Shiites in Baghdad and elsewhere, Karbala is usually much more secure.

- 1,300 years of mourning -

IS claimed a Sunday bombing in Baghdad that hit a tent where Shiites were distributing refreshments on the occasion of the commemorations for Imam Hussein, killing at least five people.

Imam Hussein was killed in 680 AD by forces of the Caliph Yazid, and his death is marked every year on the 10th day of Muharram, the first month of the Islamic calendar, which is known as Ashura.

Imam Hussein's death was part of a dispute over who should succeed the Prophet Mohammed, which eventually developed into a bitter schism between the Sunni and Shiite branches of Islam.

Some Muslims, who became known as Shiites, believed that a blood relative of the Prophet Mohammed should succeed him as the spiritual and temporal leader of Muslims, and backed his cousin and son-in-law Ali -- Hussein's father -- as successor.

Others, now known as Sunnis, insisted that relationship to the prophet by blood was not required -- a position that carried the day for his three immediate successors before Imam Ali became the fourth.

Muawiyah, who founded the Umayyad dynasty, took power as caliph on Ali's death, and, according to Shiite tradition, named his son as successor in violation of an agreement under which Imam Hussein should have succeeded.

According to Shiite belief, Imam Hussein went knowingly to his death at the hands of Yazid's forces in what is now Iraq in a bid to expose the corruption and irreligiosity of his rule.

This ideal of self-sacrifice is a key tenet of Shiite Islam to this day, inspiring followers to give their lives for causes, including the war against IS.

Iraqi forces are currently preparing for a final push on Mosul, the last city in Iraq held by IS, which has lost much of the ground it seized in 2014.