Baghdad, Iraq ? The Iraqi capital suffered its worst day of carnage in more than a month Tuesday as car bombs, roadside explosives and gunfire killed more than 100 people throughout the capital.

Coming one day after a botched execution severed the head of Saddam Hussein’s half brother and on the ninth day of a joint U.S.-Iraqi assault that has decimated the Sunni Muslim Haifa Street neighborhood, the violence heightened the already tense mood of a city wracked by war and girding for the arrival of more U.S. and Iraqi troops.

Fewer than 4,000 of the 17,500 additional troops President Bush has ordered to Baghdad have arrived in the capital, U.S. officials said Monday, and Tuesday was a reminder of how difficult their job of reining in sectarian violence will be. Bombs hit a crowded university area, a marketplace and a central Baghdad neighborhood. Gunmen later opened fire on pedestrians at a market in an eastern Baghdad district.

Mahdi Army militiamen, who’d become scarce in recent days, were back in force on the outskirts of Sadr City, setting up dozens of checkpoints. Ambulances streamed into Sadr City, the militia’s stronghold. Militiamen said the vehicles were carrying weapons, not wounded, to prepare for any U.S.-led military assault. U.S. military convoys could be seen throughout the capital.

Iraqi Prime Minister Nouri al-Maliki, a Shiite, blamed “terrorists” and “Saddamists” for the violence, but there was no way to know for sure who was behind the bloodshed. Most of the attacks took place in Shiite-controlled neighborhoods, but at least one was in a predominantly Sunni area.

The worst bloodshed came at Mustansiriyah University in a Mahdi Army-controlled area near Sadr City. A car containing a bomb drove up to the main gate as students gathered to board minivans home following afternoon classes.

One man searched for his son and finally found his head and torso but no legs.

“Where is his other half?” he asked and then shook with violent sobs.

The bombing killed 70 people and injured 169, according to the Ministry of Interior. State television issued an urgent plea for blood donations.

The university bombing occurred at 4:30 in the afternoon. But violence had begun hours earlier.

In the Shiite area of Karrada, in central Baghdad, a roadside bomb exploded at 10:30 a.m., killing one police officer and two bystanders.

At 1 p.m. two bombs exploded near a motorcycle shop in the mostly Sunni neighborhood of Bab al Sheikh, killing at least 15 people and injuring 70 others.

At 3 p.m., a bomb in a minibus exploded in Sadr City, a vast Shiite slum, killing six and wounding 11. The explosion took place a few hundred feet from an office belonging to Muqtada al-Sadr.

The last major attack came at 4:45 p.m., when gunmen opened fire on a market in the Binog neighborhood of eastern Baghdad, killing 12 civilians and wounding 17.

Al-Maliki condemned the university bombing. “They committed a shameful stain upon humanity,” he said in a statement. “Those who did it will not escape from punishment and the hand of justice will chase them no matter how long it takes.”

The mayhem came on the same day that the United Nations announced that 34,452 civilians died in violence in Iraq during 2006. The U.N. report said the “root cause of the sectarian violence lie in revenge killings and lack of accountability.”