Three bombs tore through crowds of Shi'ite pilgrims celebrating a major ritual in Iraq's Hilla city on Monday, killing least 22 - mostly women and children - and wounding 60 more, local police and witnesses said.

The attacks, at the height of Ashura, which commemorates the death of Prophet Mohammad's grandson Imam Hussein and defines Shi'ite Islam, underscored Iraq's fragile security as the last U.S. troops withdraw from the country by the end of the year.

In the first attack, a car bomb blasted the end of one Shi'ite procession, killing 16 mainly women and children, wounding 45 others and leaving bloody pools, shoes and tore clothes scattered across the street, police and witnesses said.

A powerful and horrible explosion went off behind us, smoke filled the area, said Hadi al-Mamouri, who was taking part in the ritual. I could only hear the screams of women and I could only see the bodies of women and children on the street.

A second attack involving two roadside bombs killed at least six more people at another procession in Hilla and wounded 15 more, police sources said.

The attacks came as the last 10,000 American troops prepare to withdraw by the end of 2011, more than eight years after the invasion that ousted Sunni dictator Saddam Hussein and allowed the country's Shi'ite majority to ascend.

On Monday, an Iraqi Sunni Muslim insurgent group with links to Hussein's banned Baath party vowed to continue attacks on U.S. personnel staying in Iraq even after troops withdraw.

Sunni Islamist insurgents often target Shi'ite shrines and ceremonies in an attempt to inflame sectarian tensions still simmering close to the surface in Iraq.

Violence has eased sharply since its worst years in 2006-2007 when Sunni and Shi'ite armed groups killed thousands in intercommunal assassinations and bombings. But Sunni insurgents and Shi'ite militias still carry out deadly attacks.

Iraq's security forces say they are generally ready to contain the stubborn insurgencies, but they acknowledge gaps in their abilities such as air defence and intelligence gathering once the American military depart.

(Reporting by Baghdad newsroom; Writing by Patrick Markey; Editing by Louise Ireland)