Shiite pilgrims converge on the holy the shrine of Imam Moussa al-Kadhim in the Kazimiyah neighborhood of north Baghdad, 09 Aug 2007

Hundreds of thousands of Iraqi Shi'ites marched to a shrine in Baghdad Thursday in an annual pilgrimage held under tight security.

Many of the pilgrims walked for days in extreme summer heat to reach a mosque in northern Baghdad where a revered Shi'ite imam Moussa Kadhim is buried. The pilgrimage commemorates the imam's death 12 centuries ago.

Iraqi authorities deployed additional police in the capital and banned vehicles from streets until Saturday to prevent attacks by Sunni insurgents during the event. Only scattered incidents of violence were reported.

Last year, snipers killed at least 20 Shi'ites in Baghdad as they walked to the shrine. At the same festival in 2005, an estimated 1,000 pilgrims died in a stampede over a bridge triggered by rumors of a suicide bomber.

In other developments, the U.S. military says coalition forces killed three terrorists and detained 21 suspects in operations in the Tigris River valley since Wednesday.

The U.S. military also says one Marine was killed in combat Tuesday in Iraq's al-Anbar province.

Britain's defense ministry says a roadside bomb killed two British soldiers and wounded two others today when it hit their convoy west of Basra, in southern Iraq.

Some information for this report was provided by AP and Reuters.