FACTBOX: What is the Islamic event of Ashura?

(Reuters) - Thousands of Shi'ites are gathering in the Iraqi city of Kerbala for Ashura, one of the holiest rituals on the Shi'ite religious calendar, which climaxes on Saturday.

Iraqi authorities have launched a major operation to protect pilgrims attending one of the world's largest religious rites, a target in the past for al Qaeda militants, which the U.S. military says is still the biggest threat to peace in Iraq.

Security for the event has been tight since suspected al Qaeda suicide bombers and mortar attacks killed 171 people during Ashura in Kerbala and in Baghdad in 2004.

Here are some facts about Ashura.

* WHAT IS ASHURA?

-- Ashura is the 10th day of the lunar month of Muharram when according to Islamic tradition Imam Hussein, grandson of the Prophet Mohammad, was killed in battle in 680.

-- It is observed in Iraq and in other countries with sizeable Shi'ite communities, including Afghanistan, Azerbaijan, Bahrain, Iran, Lebanon, Pakistan, Saudi Arabia and Syria. Several million pilgrims are expected to converge on Kerbala.

-- Sunni Muslims also mourn Hussein's death, but less fervently, and Ashura is a reminder of Sunni-Shi'ite rifts dating from disputes over the Prophet Mohammad's succession.

* THE BATTLE: