Thousands of Iraqi police and troops are on high alert in the Shi'ite holy city of Karbala as hundreds of thousands of pilgrims gathered for a Shi'ite religious festival.

Officials closed the city to cars Friday to prevent car bomb attacks. North of Karbala, two car bombers killed eight people, including seven police officers, on a road linking the city to Baghdad.

Celebrations begin Monday marking the birthday more than 11 centuries ago of a revered Shi'ite leader, Imam Mehdi. Pilgrims are pouring in even though al-Qaida in Iraq leader Abu Musab al-Zarqawi declared war on Shi'ites last week.

Further south, two Iraqis were killed in violence that erupted after Iraqi authorities detained two British soldiers in Basra.

In a separate development, one of Saddam Hussein's nephews was sentenced to life in prison for bomb-making and funding the country's insurgency.

Some information for this report provided by AFP and Reuters.