Police and witnesses said the blasts were caused by explosives planted near holy sites in the southern city of Karbala and the Kazimiya shrine in Baghdad, though some people blamed suicide bombers.

The attacks sparked a wave of Shiite outrage - much of it directed at U.S. troops in the Iraqi capital. American soldiers who arrived at Kazimiya were attacked by angry crowds throwing stones and garbage, injuring two Americans.

U.S. intelligence officials have long been concerned about the possibility of militant attacks during the Shiite festival of Ashoura. Last month, U.S. officials released what they said was a letter by Jordanian militant Abu Mousab al-Zarqawi outlining a strategy of spectacular attacks on Shiites, aimed at sparking a Sunni-Shiite civil war.

Also Tuesday, insurgents threw a grenade into a Army Humvee as it drove down a Baghdad road, killing one 1st Armored Division soldier and wounding a second. The death brings to 548 the number of U.S. service members who have died since the United States launched the Iraq war in March. Most have died since President Bush declared an end to active combat May 1.

At least 31 people were killed and 100 wounded in the Karbala attacks, police officer Muhammad Saad said. In the Iraqi capital, four hospitals reported 52 killed in the Baghdad blasts, though an unknown number of victims were taken to other facilities. Deputy Interior Minister Ahmed Kadhum Ibrahim reported 56 dead in Baghdad.

In Karbala, 50 miles south of Baghdad, five large blasts went off shortly after 10 a.m. near the golden-domed shrine of Imam Hussein, one of Shiite Islam's most beloved saints, and another shrine. The explosions hurled bodies in all directions and sending crowds of pilgrims fleeing in panic.

Dead and wounded were loaded onto wooden carts normally used to ferry elderly pilgrims to holy sites. Bodies ripped apart by the force of the blasts lay on the streets.

At about the same time, three explosions went off inside and outside of Baghdad's Kazimiya shrine, which contains the tombs of two other saints. Panicked men and women, dressed in black, fled screaming and weeping as ambulances raced to the scene.

Crowds of enraged survivors swarmed nearby hospitals, some blaming Americans for stirring up religious tensions by launching the war, others blaming al-Qaida or Sunni extremists.

Stone-throwing Iraqis attacked Army medics trying to help wounded at Kazimiya, driving the U.S. troops back into their high-walled compound then trying to storm the gates. Soldiers threw smoke grenades and fired shotguns into the air to drive the mob off.

The Ashoura festival, which marks the killing of Hussein in a 7th century battle, is the most important religious period in Shiite Islam and draws hundreds of thousands of pilgrims from Iraq, Iran, Pakistan and other Shiite communities to the Iraqi shrines.

Iran condemned the blasts as "terrorist" and "vicious" attacks, according to Iranian state radio. Iranian Foreign Ministry spokesman Hamid Reza Asefi said the United States and its allies are "responsible for security" for the pilgrims at Karbala and in Baghdad.

In Beirut, a spokesman for Iraq's leading Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani, blamed American soldiers for the attacks, saying they were responsible for the security. Sheik Hamed Khafaf said U.S. officials had ignored repeated requests to bolster security for the pilgrims.

"Those behind this painful incident have no links with Islam... The criminal who target innocents and Muslims in this holy place and on this pure land is not a Muslim," said Ali Abdul-Karim al-Safi al-Musawi, an al-Sistani representative in Basra, in southern Iraq.

There were contradictory reports on the cause of the blasts. The U.S. military initially said four mortars struck around the Kazimiya shrine, but a spokesman later said it was not certain that mortars were to blame.

Ibrahim, the deputy interior minister, said the blast at Kazimiya was caused by bombs, not mortars, and witnesses in Karbala also blamed planted explosives or suicide bombers.

One witness in Karbala, identifying himself only as Sairouz, said a bomb buried under rubbish went about a dozen yards from him. "Many Iranians were killed," he said.

"We were standing there (next to the mosques) when we heard an explosion. We saw flesh, arms legs, more flesh. Then the ambulance came," said Tarar, an 18-year-old, giving only one name.

The Kazimiya blasts went off inside the shrine's ornately tiled walls and outside in a square packed with street vendors catering to pilgrims. The courtyard inside was strewn with torn limbs. The street outside was littered with picnic baskets brought by pilgrims and thousands of shoes and sandals belonging to worshippers who had been praying inside.

Hundreds of gunmen swarmed around the shrine, and a U.S. helicopter hovered overhead. Black mourning banners traditional in Ashoura celebrations hung in tatters. Posters of prominent Shiite clerics were stained with blood.

"The Jews and the occupation troops are behind these blasts," a voice on one loudspeaker blared.

In the southern city Najaf, near Karbala, police Monday night found and defused a bomb hidden near the shrine of Imam Ali, the most important Shiite saint, Iraqi Police Capt. Imad Hussein said.

The Najaf shrine was attacked on Aug. 29 by a massive car bomb that killed more than 85 people, including Shiite leader Ayatollah Mohammed Baqir al-Hakim.

In the letter released by the U.S. military last month, al-Zarqawi, an extremist believed linked to al-Qaida, wrote that stepped up attacks were needed to disrupt the planned handover of power to the Iraqis on June 30.

Mouwafak al-Rubaie, a Shiite member of the Iraqi governing council, told CNN that Tuesday's attacks bore al-Zarqawi's fingerprints.

"This is a message from Zarqawi to the Iraqi people and we received the message. It is written in blood now," al-Rubaie said after visiting the Baghdad shrine.

Also Tuesday, a land mine exploded in the Abu Nawas neighborhood of Baghdad, damaging a car used by the Arab television station Al-Jazeera and lightly wounding several staffers.