Witnesses said a woman carried out the attack at the business school annex to Mustansiriyah University, but Interior Ministry officials said it was investigating the reports. The school's main campus was hit by a string of bombings last month that killed 70 people.

The attack came as a powerful Shiite militia leader bitterly complained that "car bombs continue to explode" despite an ongoing security crackdown in Baghdad and suggested he was rethinking his cooperation.

The statement issued in the name of the radical cleric Muqtada al-Sadr put increased strains on the U.S.-Iraqi security sweeps ¡ª aimed at restoring order in the capital. The cleric said any crackdown that includes American soldiers was doomed to failure.

Al-Sadr's Mahdi Army militia pulled its fighters off the streets under intense government pressure to let the 12-day-old security plan proceed. But a relentless wave of Sunni attacks ¡ª six alone in the Baghdad area Sunday ¡ª has apparently tested al-Sadr's patience as well as many ordinary Shiites.

A return to the streets by the Mahdi Army forces could effectively block the security effort and raise the chances of Baghdad falling into sectarian street battles ¡ª the apparent aim of Sunni extremists seeking any way to destroy the U.S.-backed government.

"Here we are, watching car bombs continue to explode to harvest thousands of innocent lives from our beloved people in the middle of a security plan controlled by an occupier," said a statement read by an al-Sadr aide in Baghdad.

Al-Sadr ¡ª who has not appeared in public in more than a month ¡ª is no friend of Washington and his forces fought fierce battles with U.S. troops in 2004. But he has largely cooperated in the Iraqi political process to avoid strains with Prime Minister Nouri al-Maliki and the Shiite leadership.

The statement, however, was highly critical of the U.S. role in the security plan and urged leaders to "make your own Iraqi (security) plans." He said "no security plan will work" with direct U.S. involvement.

The statement ¡ª read to hundreds of cheering supporters ¡ª was the first public word from the al-Sadr since U.S. assertions earlier this month that he fled to neighboring Iran to avoid arrest. Al-Sadr's aides and other loyalists insist he never left Iraq.

The political situation in Iraq was further thrown into question after President Jalal Talabani, a 73-year-old Kurd, was taken to Jordan for medical tests after feeling ill because of "continuing hard work over the past few days," his office said.

Talabani's son, Qubad Talabani, told CNN his father was suffering from fatigue and exhaustion. "He did not have a heart attack" or a stroke, he said. "He's absolutely up and about, being able to communicate."

A doctor said Talabani was being treated at the heart center at King Hussein Medical City in Amman because the facility has modern equipment, not necessarily because the president suffers from a heart ailment.

Shiite anger at the United States is running high since American soldiers on Friday detained the son of the most powerful Shiite political leader for nearly 12 hours after he crossed from Iran. U.S. officials claim Shiite groups, including the Mahdi Army, receive weapons and aid from Iran. Iran denies the charges.

"To my Shiite and Sunni brothers, I say, `Let us scorn sectarianism and hoist the banner of unity,'" said the statement from al-Sadr, whose militia is blamed for frequent execution-style slayings of Sunni rivals.

Since the security crackdown began, the number of bodies thought to be victims of Shiite death squads has gone down dramatically in Baghdad, but there has been no respite from violence blamed on Sunni insurgents.

Besides the college blast, at least 18 people were killed ¡ª mostly in Shiite districts ¡ª in bombings and rocket attacks in the Baghdad area.

Security guards at the Mustansiriyah University annex scuffled with the bomber before the blast, witnesses said. Most of the victims were students, including at least 46 injured, said police

If the bomber is found to be woman it would unusual ¡ª but not unprecedented ¡ª in Iraq's chaos. The main campus at Mustansiriyah, about 1 1/2 miles away, was the target of twin car bombs and a suicide blast last month that killed 70 people.

The students at the business college were returning to midterm exams after the Iraqi weekend.

A 22-year-old student, Muhanad Nasir, said he saw a commotion at the gate. "Then there was an explosion. I did not feel anything for 15 minutes and when I returned to consciousness, I found myself in the hospital," said Nasir, wounded in the head and chest.

The blast flung blood-soaked notebooks and backpacks among the lifeless bodies and wounded. Cement walls were pockmarked by the hail of ball bearings. Parents rushed to the site and some collapsed in tears after learning their children were killed or injured. Students used rags and towels to try to mop up the blood.

The school is located in a mostly Shiite district of northeast Baghdad, but does not limit enrollment to that group.

In the northern city of Mosul, U.S. troops killed two gunmen in a raid and captured a suspected local leader of the insurgent group al-Qaida in Iraq, the military said. Additional details were no immediately available.

Iraq's Interior Ministry, meanwhile, raised the toll from a suicide truck bombing in the violence-wracked Anbar province on Saturday to 52 dead and 74 injured.

The attack on worshippers leaving a mosque in Habbaniyah, about 50 miles west of Baghdad, was believed linked to escalating internal Sunni battles between insurgents and those who oppose them.

"This cowardly act of violence underscores that the terrorists are the enemies of all Iraqis, regardless of sect," the U.S. ambassador to Iraq, Zalmay Khalilzad, said in a statement. "They want Iraq to fail. Now is the time for the Iraqis to come together against these terrorists."