KARBALA, Iraq (AP) -- A series of co-ordinated blasts struck major Shiite Muslim shrines in Karbala, Iraq and in the capital Baghdad on Tuesday as thousands of pilgrims converged on the climactic day of the sect's most important religious festival. Scores were killed and wounded, witnesses said. Arab television stations reported 25 dead in Karbala. One hospital reported 18 killed in the Baghdad blast but an unknown number of victims were also taken to other facilities. Stunned witnesses believed the attacks were carried out by suicide bombers or planted explosives. U.S. intelligence officials have long been concerned about the possibility of militant attack on the Ashoura festival and coalition and Iraqi forces bolstered security around Karbala and other Shiite-majority towns in the south during the pilgrimage. Last month, U.S. officials released what they said was a letter by a Jordanian militant outlining a strategy of spectacular attacks on Shiites, aimed at sparking a Sunni-Shiite civil war. Also Tuesday, insurgents threw a grenade into a U.S. army Humvee as it drove down a Baghdad road, killing one 1st Armored Division soldier and wounding another.

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 George W. Bush declared an end to active combat May 1. In Karbala, 80 kilometres south of Baghdad, five large blasts went off shortly after 10 a.m. local time near two of the most important shrines in Shiite Islam, hurling bodies in all directions and sending crowds of pilgrims fleeing in panic. Ten bodies that appeared to be dead were loaded onto wooden carts and taken away. Bodies ripped apart by the force of the blasts lay on the streets. There were varying accounts on the cause of the explosions in Karbala, where Polish troops are in charge of security. An Iraqi police spokesman said the blasts were caused by suicide attackers, wounding at least 300 people, the Polish news agency PAP reported. But Col. Zdzislaw Gnatowski, a Polish military spokesman in Warsaw, blamed a series of mortars fired into the shrine area. At about the same time, three explosions rocked the inside and outside of the Kazimiya shrine in Baghdad. Panicked men and women, dressed in black, fled screaming and weeping as ambulances raced to the scene. Angry mobs hurled stones at U.S. troops who later pulled into the square outside Kazimiya in Humvees and an armoured vehicle. 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. Some witnesses at Kazimiya said the blasts were carried out by suicide bombers. The Kazimiya shrine in northern Baghdad contains the tombs of two other Shiite saints, Imam Mousa Kazem and his grandson Imam Muhammad al-Jawad. The Ashoura festival, which marks the 7th century killing of Imam Hussein, 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. In the Lebanese capital Beirut, a spokesman for Iraq's leading Shiite cleric, Grand Ayatollah Ali al-Husseini al-Sistani, blamed U.S. soldiers for the attacks, saying they were responsible for the security. Sheik Hamed Khafaf said U.S. officials ignored repeated requests to bolster security for the pilgrims. Shiite cleric Sheik Sayyed Akeel al-Khatib said the explosions, "especially those at Kazimiya," were perpetrated by suicide bombers. "These means they came from abroad and were not were Iraqis," he told Lebanese Broadcasting Corp. TV. The Karbala blasts struck near the golden-domed shrine where Imam Hussein is buried, in a neighbourhood of several pilgrimage sites. After the blasts, Shiite militiamen tried to clear the terrified crowds, firing guns into the air. Two more blasts went off about a half-hour later. "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. Two armed Iraqi policemen broke down in tears as they walked through the bomb site. Iraqi militia initially tried to control the crowd and arrested two men the crowd attempted to lynch. Rumours swirled throughout the city as to the cause of the blasts, ranging from mortars fired from outside the town to suicide bombers in the crowd. One witness said a bomb was hidden near the mosque. "Many Iranians were killed, I was 10 metres away, it was hidden under rubbish," one witness, identifying himself only as Sairouz, said. 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 street outside Kazimiya was littered with picnic baskets brought by pilgrims and thousands of shoes and sandals belonging to worshippers who had been praying inside the shrine. The courtyard inside the shrine was strewn with torn limbs. Hundreds of gunmen swarmed inside and outside the walled shrine as men wept. A U.S. helicopter hovered over the shrine. Black mourning banners traditional in Ashoura celebrations hung in tatters.