Hundreds of thousands of Shiites descended Monday on the holy city of Karbala (search), 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 (search), Mohammed al-Mahdi.

Similar gatherings in Karbala (search) 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 Qaeda in Iraq leader Abu Musab al-Zarqawi (search), in a recorded message, declared an "all-out war" on Shiites and others deemed American collaborators. But Iraqi Vice President Adil Abdul-Mahdi (search) said Monday such threats would only "strengthen the defiance" of Iraqis.

Abdul-Mahdi said the Shiite pilgrims heading to Karbala were the "biggest [example of] defiance in the face of threats by al-Zarqawi," according a statement released by the Iraqi presidency.

The explosives cache found near Karbala was discovered late Sunday on farm land on the outskirts of the city, said police spokesman Rahman Mishawi.

Three "non-Iraqi Arabs" were arrested, Mishawi said, along with a man armed with several hand grenades who was caught walking with a procession of pilgrims.

Authorities were taking no chances this time.

Karbala police Brig. Gen. Mohammed al-Hasnawi said that the city had been closed off to all vehicles since Friday, and that about 6,000 police and Iraqi army troops were deployed throughout the city.

Policewomen would search female pilgrims, and nonresidents were required to obtain prior approval from authorities before being allowed to check in to hotels, he told The Associated Press.

Officials said they expected more than 1 million people to gather for the celebration marking Imam al-Mahdi's birth in 868 A.D. According to Shiite tradition, he vanished without a trace and will return before Judgment Day leading the faithful in a battle for a true and just Islamic state.

Additional security at holy sites was to be provided by the al-Mahdi Army (search), a militia loyal to the radical Shiite cleric Muqtada al-Sadr (search), as well as by the Badr Brigade (search), a rival militia linked to the Supreme Council for the Islamic Revolution in Iraq (search), another major Shiite party.

Government officials had not announced that Saddam's nephew, Ayman Sabawi (search), was on trial before revealing that he had been convicted and sentenced to life in prison "for funding militants and possessing and manufacturing roadside bombs."

Sabawi, the son of Saddam's half brother Sabawi Ibrahim al-Hassan (search), who served as a presidential adviser before the U.S.-led invasion, was captured in early May.

The Iraqi Central Criminal Court in Baghdad said Sabawi would face a second trial — beginning Nov. 1 — for other, unspecified additional crimes to which he allegedly confessed during pretrial interrogation.

His colleague, Tareq Khalaf Mizal (search), arrested along with Sabawi, was sentenced to six years.

Since Wednesday, when 14 suicide bombs exploded in Baghdad in the bloodiest day in the capital since the war began, a staggering wave of insurgent violence has killed at least 250 people and wounded hundreds more nationwide.

Shiites have suffered the brunt of the attacks, which al-Zarqawi said were in retaliation for the Iraqi-U.S. military operation against the insurgent stronghold of Tal Afar (search), a northern city near the Syrian border.

The operation began Sept. 10, when a force of 5,000 Iraqi soldiers backed by 3,500 Americans stormed the city to clean it of insurgents for the second time in a year. Mopping-up operations continued Sunday with the Iraqi military reporting a total of 157 insurgents killed and 440 captured during the 10-day offensive.

Army spokesman Maj. Gen. Abdul Aziz Mohamed-Jassim, said nine Iraqi soldiers and six policemen had died. The U.S. military has reported no deaths among its forces in the ancient city, known for its old citadel.