BAGHDAD, Iraq -- A suicide bomber pushed past guards at a crowded college campus Sunday and set off a thunderous blast that killed at least 40 Iraqis, most of them female students who were waiting in line in the midday sun to enter classrooms for midterm exams.

The attack was the second in recent weeks to target the mainly Shiite Mustansiriya University, and it sent a clear message that whatever calm had followed the launch of the latest U.S.-Iraqi security plan was over. Even as rescue workers mopped blood from the college grounds and as the wounded told their stories of survival, the Iraqi government insisted the plan launched nearly two weeks ago was succeeding.

But radical Shiite cleric Muqtada al-Sadr, whose followers virtually control the campus, denounced the plan as a failure and said Iraqi government troops and police should take charge of security and "invaders," a reference to U.S. troops, should leave.

Most Sunni Arabs distrust the Iraqi security forces because they are dominated by Shiites, and they also accuse the Shiite-led Iraqi government of not doing enough to rein in al-Sadr's militia. In particular, they note that coalition troops have yet to move into the Shiite slum of Sadr City in large numbers, despite Prime Minister Nouri al-Maliki's vow that the security plan, which aims to put thousands more U.S. and Iraqi troops on Baghdad's streets, would target Shiite as well as Sunni areas.

Al-Sadr's comments, however, suggested that he too is fed up with the plan. The anti-American firebrand had agreed to cooperate with al-Maliki by drawing down his militia forces when the plan was launched, and the result may have been evident in the decreased number of Sunni victims of Shiite death squads found along Baghdad's streets in recent weeks.

Mustansiriya University's main campus and its satellite colleges are sandwiched between Sadr City and a mainly Sunni area. Students say al-Sadr's Mahdi Army militia virtually controls the university, enforcing conservative dress codes for women and canceling classes to honor Shiite martyrs.

The militia's presence has made Mustansiriya a sectarian battleground that has served as a ripe target for Sunni attackers. On Jan. 16, at least 70 students were killed when two car bombs exploded virtually simultaneously on the university's main campus, about a mile and a half from the College of Business Administration and Economics, where Sunday's attack took place.

The bomber Sunday struck at an especially busy time at the college gate. Two lines, one for female students and one for male students, had formed as students waited to be checked by guards, who patted them down, looked inside their bags, and confiscated cell phones and other items that could hide explosives.

As they were being allowed in for 1 p.m. mid-terms, other students who had sat for morning exams were filtering out or milling in the campus courtyard comparing notes.

One person resisted being searched. The guards became agitated.

Lu'ay Sadek, a business management student who is Shiite, heard yelling near the gate and moved closer to see what was happening. As he did, the bomb detonated, sending shrapnel and flames across the campus. Sadek saw a wall of fire and the bodies of guards and classmates flying into the air. "Those are the same people I talk to everyday," said Sadek, who woke up in Imam Ali Hospital after having lost consciousness.

The words of the wounded underscored the despair and bewilderment of both Sunni and Shiite students as their attempts to do something as routine as take midterms were upended by sectarian and ideological extremism.

"The last thing I expected was that students would be attacked, especially because we are in the middle of a residential area," said Saad Abdalla, a Sunni economics student, who suffered burns and shrapnel wounds to his head. "We felt safe, especially after the application of the new security plan."

Most of the victims were young women, because the female students' line was far longer than the men's line, said student Muaataz Jawad, explaining that it had taken longer to check female students because of their handbags.

Jawad said surviving guards told him the bomber was a female, but other witnesses and police said it was a man who wore the explosives strapped around his torso. No group immediately claimed responsibility for the attack, but it was similar to others on mainly Shiite targets, such as crowded markets, which have appeared designed by Sunni insurgents to inflict maximum suffering.

The university bombing overshadowed a string of smaller attacks across Iraq on Sunday. Mortar attacks and car bombs killed at least three civilians in Baghdad, and a roadside bomb in Kirkuk killed one person.

Also Sunday, the Iraqi government announced that President Jalal Talabani had been flown to Jordan after falling ill with "malaise" as a result of overwork. A brief statement from Talabani's office said doctors had recommended additional tests but that there was no cause for concern.

U.S. military officials said they had killed two suspected terrorists and captured a senior al-Qaida leader on Sunday during a raid in Mosul. Five other suspected insurgents also were detained in the raid, the military said in a statement.

Times staff writer Saif Hameed and special correspondents in Baghdad contributed to this report.