FACTBOX: Shi'ite Ashura procession attacked in Pakistan

KARACHI (Reuters) - A suicide bomber attacked a Shi'ite Muslim procession in Pakistan's commercial capital Karachi on Monday, killing at least 25 people, officials said.

Ashura, the Shi'ite calendar's biggest event, has become a flashpoint for deadly attacks by Sunni militants over recent years.

Here are some facts about Ashura and frayed relations between Pakistan's Sunni and Shi'ite groups:

WHAT IS ASHURA?

Ashura falls on the tenth day of a 40-day mourning period during the Islamic calendar's first month, Moharram, which commemorates the death of Imam Hussein, a grandson of the Prophet Mohammad, who was killed in battle in 680 in the Iraqi city of Kerbala.

-- The schism between Sunnis and Shi'ites is rooted in a dispute over the successors of the Prophet.

-- Sunnis regard Abu Bakr, one of Mohammad's companions, as his successor. Shi'ites revere Ali, the prophet's son-in-law and cousin. Ali was also the father of Hussein, whose death Ashura commemorates.

-- Marching down streets, worshippers flog themselves with steel-tipped flails or slash their bodies with knives to express solidarity with Hussein.

WHY IS THE FESTIVAL A FLASHPOINT FOR VIOLENCE?:

-- The most important festival in the Shi'ite calendar, it draws thousands of people to the streets in cities across Pakistan.

-- Shi'ites make up about 15 percent of Pakistan's 160 million people. The overwhelming majority are Sunni Muslims.

-- Security is usually stepped up in the days leading up to Ashura. But most attacks are carried out by suicide bombers who are almost impossible to stop.

-- In 2006, about 40 people were killed in a suicide attack on an Ashura procession in the northwestern town of Hangu. During Ashura in 2003, at least 57 Shi'ites were killed in Quetta, the capital of the western province of Baluchistan.

-- Over the past three decades, thousands of Pakistanis have been killed in feuding that sprang out of the radicalization of the two sects as a result of the jihad, or holy war, covertly funded by the United States and Saudi Arabia, against the Soviet occupation of Afghanistan, and a revolution that turned Iran into a republic led by Shi'ite clerics.

-- As well as Pakistan, there have been deadly attacks during Ashura in Iraq, which has a large Shi'ite population. Kerbala and Baghdad have seen bloody violence over the years.

(Editing by Robert Birsel)