Meanwhile, the U.S. military said it has found ingredients and equipment that Iraq may have been using to make chemical weapons. And Jay Garner, the retired American general overseeing the rebuilding of Iraq, visited the northern Kurdish region.
Muhammad Hamza al-Zubaydi, who was captured Monday by the Iraqi opposition and turned over to American authorities for trial on war-crimes charges, is the highest-ranking figure on the U.S. military's most-wanted list to be caught so far.
Al-Zubaydi was known as Saddam's "Shiite Thug" for his role in Iraq's bloody suppression of the Shiite Muslim uprising of 1991. Tens of thousands of people died in the revolt. Iraqi opposition groups have also accused al-Zubaydi of the 1999 assassination of a top Shiite cleric.
A former prime minister, al-Zubaydi was No. 18 - the queen of spades - in the U.S. military's 55-card deck of most-wanted regime figures.
Al-Zubaydi was considered one of the most brutal figures in the regime and was listed in a U.S. State Department report titled "Iraq: Crimes Against Humanity, Leaders as Executioners." A Shiite himself, he was once featured in an Iraqi videotape brutalizing Shiite dissidents.
"This is very significant - he is one of the most hated men in the former regime," said Haider Ahmad, a spokesman for the Iraqi National Congress, the leading exile group.
The INC said it arrested al-Zubaydi hiding out in Hillah, a town about 60 miles south of Baghdad.
Eight of the 55 most-wanted are now in custody. A ninth figure, Ali Hassan al-Majid - known as "Chemical Ali" for his use of poison gas against Iraq's Kurds - is believed to have been killed in an airstrike.
Shiites, meanwhile, streamed toward Najaf - burial shrine of Imam Ali, son-in-law of the Prophet Muhammad - and Karbala, where Hussein, Muhammad's grandson, was martyred in the 7th century. Up to 2 million Shiites are expected to take part in the annual pilgrimage, which culminates on Thursday.
During Saddam's rule, the Shiites were forbidden to march. This year, they marched openly under black banners of mourning.
"We were prohibited from visiting these shrines for a long time by the Baath Party and their agents," Abed Ali Ghilan said in Karbala. "This year we thank God for ridding us of the dictator Saddam Hussein and for letting us visit these shrines."
Karbala was packed shoulder-to-shoulder with hundreds of thousands, if not millions, of pilgrims, waving black and green flags, chanting and beating their chests. Others carried photos of famous Shiite clerics.
Men wore mostly white robes; women were cloaked head to toe in traditional black dress.
Two groups of 100 men in white robes slashed open their own heads with swords, splattering blood on those nearby. Waving the bloody blades toward the shrine and screaming with joy, some were taken away in cars for medical attention. Others washed up at a traditional Iraqi bathhouse.
Water trucks were brought in to help the crowds endure the 90-degree heat and blazing sun. Roving watermen sprayed the masses to keep them cool.
The portrait of Saddam that used to hang on the main Imam Hussein shrine was gone, underlining the new sense of freedom enjoyed by Iraqi Shiites.
The latest reported discovery by U.S. weapons experts could help prove the existence of the weapons of mass destruction that President Bush cited as the reason for going the war.
The find was made several days ago with the help of an Iraqi scientist who claimed to have worked in Saddam's chemical weapons program.
U.S. military officials refused to name the scientist or identify the material, which had been buried in the ground. Many chemical weapons ingredients have nonmilitary purposes and officials cautioned that the findings, which are being analyzed, do not confirm the presence of chemical weapons.
According to the officials, who spoke on condition of anonymity, the scientist said the Iraqis set a warehouse on fire and destroyed chemical weapons and biological warfare equipment days before the war began March 20.
The scientist also said that several months before the war, he watched as Iraqi officials buried chemical precursors for weapons and other sensitive material to conceal and protect them.
Chief U.N. weapons inspector Hans Blix cautioned the Americans to "examine everything critically," noting that some Iraqis may be motivated to claim more than they know.
The U.S. government is sending more than 1,000 experts specializing in weapons, intelligence and computers to join the search for weapons of mass destruction. They will analyze documents, interrogate prisoners and scour suspicious sites, joining some 200 experts already on the hunt.
Also Tuesday, Garner, Iraq's civilian administrator, visited the Kurdish north on his second day of a tour around the country. He arrived in Baghdad on Monday to take up his duties.
Garner's helicopter landed under heavy guard in Sulaymaniyah, where he was met by a delegation from the Patriotic Union of Kurdistan, one of the groups jockeying for power in postwar Iraq.
In other developments: