KARBALA, Iraq - Hundreds of thousands of Shiites descended today on the holy city of Karbala, paying tribute to one of their most revered religious figures in what Iraqi officials called a defiant pilgrimage despite threats by militants.

In Baghdad, officials also announced that one of Saddam Hussein's nephews was sentenced to life in prison for funding the violent insurgency, and would stand trial on other charges.

Authorities said they had already uncovered a cache of explosives and arrested four insurgents for allegedly planning on the pilgrims attending festivities marking the birthday of the 12th Shiite imam, Mohammed al-Mahdi.

Similar gatherings in Karbala and other Shiite holy cities have ended in tragedy. In December, more than 50 pilgrims were killed in a series of bombings in Karbala, 50 miles south of Baghdad, and in March, at least 181 people died in coordinated bombings of Shiite pilgrims in Karbala and Baghdad. Both attacks were blamed on Sunni extremists.

Just days before this celebration, al-Qaida in Iraq leader Abu Musab al-Zarqawi, in a recorded message, declared an "all-out war" on Shiites and others deemed American collaborators. But Iraqi Vice President Adil Abdul-Mahdi said today that such threats would only "strengthen the defiance" of Iraqis.