"I saw people falling over, some of them blown apart," he said.

Tayaran Square is near several government ministries and a bridge that crosses the Tigris River to the heavily fortified Green Zone, where Iraq's parliament and the US and British embassies are located.

Most of the victims were Shiites from poor areas of the capital such as Sadr City, said police Lt. Bilal Ali.

Parliament speaker Mahmoud al-Mashhadani, a Sunni, said the attack targeted poor people who were trying to feed their families, "turning them into pieces of flesh" and urged the deeply divided legislature of Shiites, Sunnis and Kurds "to find a solution" to Iraq's many problems.

Prime Minister Nouri al-Maliki, a Shiite, blamed the "horrible crime" on Sunni extremists. "Iraq's security forces will chase the criminals and bring them to justice," he said.

The prime minister has come under fire from powerful politicians who accuse him of not doing enough to stop the sectarian violence that has spiraled since a Feb. 22 bombing destroyed the golden dome of a revered Shiite mosque in Samarra.

On Monday, police discovered a bomb hidden outside the Samarra mosque. The bomb exploded while coalition bomb disposal officials were removing it, slightly damaging the building, the US command said.

Al-Maliki said Tuesday there was no alternative to his national unity government, arguing that talks between major partners in his ruling coalition to form a new bloc in parliament did not amount to a bid to unseat him.

Two key political figures, Vice President Tariq al-Hashemi, a Sunni who was meeting with President Bush on Tuesday in Washington, and senior Shiite leader Abdul-Aziz al-Hakim, have been linked to behind-the-scenes efforts to form a new bloc in parliament that would replace the alliance now supporting al-Maliki's government.

"What is going on now is positive when the aim, contrary to what has been said, is to broaden the government's political base and not an attempt to undermine its ideology or to search for alternatives," al-Maliki told reporters in his first public comments on these efforts.

"There is no alternative in Iraq for this national unity government because it is the guarantee for the political process to continue," he said.

The White House has denied that any plans were afoot to dump al-Maliki, whose seven months in office have been defined by failure to curb growing violence and improve the lives of Iraqis.

The US command announced the five more deaths of US troops in Iraq, including three Marines killed in combat in Anbar province.

The three Marines assigned to 3rd Marine Aircraft Wing died Monday of wounds sustained while fighting insurgents, according to a statement.

The US military said those killed were not linked to a hard landing Monday by a Marine helicopter in Anbar. At least 18 people were injured in that incident but hostile fire did not appear to be the cause, the military said.

Another Marine died Monday from non-hostile causes in Anbar, which stretches west from Baghdad to the borders of Syria, Jordan and Saudi Arabia.

The military also said a soldier died Monday of apparent natural causes near Diwaniyah, 80 miles south of Baghdad. The soldier lost consciousness and died after being transported to a troop medical clinic.

The American deaths raised to 51 the number of US troops who have died this month. At least 2,939 members of the US military have died since the war began in 2003, according to an AP count.

The slain AP cameraman was Aswan Ahmed Lutfallah, 35, the third Associated Press employee killed in the Iraq war. Before the death, Reporters Without Borders had recorded at least 93 journalists killed in Iraq since the war started nearly four years ago. The Committee to Protect Journalists put the number of journalists killed in Iraq at 89.

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