The bombing, which injured at least 100 people, was the deadliest attack to date on international troops who have come to Iraq at the request of President Bush to aid in U.S.-led reconstruction efforts.
Five young Iraqi women who were students at a local teachers college were burned to death when their car was incinerated. At least 18 Italian citizens, including the police and four soldiers, died. The infant who was killed was in its mother's arms in a nearby house when the explosion occurred. The blast severed the mother's arm.
Coalition authorities said that they did not know who carried out the attack but that it bore the stamp of Islamic extremists, possibly linked to the al-Qaida terrorist network, who officials believe are behind a wave of high-profile suicide bombings in Iraq during the last three months.
Since the fall of Saddam Hussein, the Nasiriyah area in the south has been one of the most peaceful regions of Iraq. Policed by Italians, Romanians and South Koreans as well as Iraqis, the area has shown little hostility toward foreign troops, and security has been more lax than in Baghdad, where most of the suicide bombings have taken place.
"This was the first explosion we heard since the end of the war," said Hassam Abdul Wahid, a security guard for the International Medical Corps, an aid organization with offices nearby. "It was like a nuclear bomb. The firestorm was so huge, huge, huge."
At the White House, President Bush mourned the Italian casualties.
"Today in Iraq, a member of NATO, Italy, lost some proud sons in the service of freedom and peace," Bush said.
"We appreciate their sacrifices," he said. "I appreciate the steadfast leadership of Prime Minister [Silvio] Berlusconi, who refuses to yield in the face of terror."
The suicide bombing took place on a day when the U.S. military launched two air attacks on insurgents in the Baghdad area and reported a firefight Tuesday outside the Jordanian-run hospital in Fallouja in which six insurgents were killed and four wounded.
The damage to the military police base in Nasiriyah was reminiscent of the 1995 bombing of the Alfred P. Murrah Federal Building in Oklahoma City. The facade and part of the side were stripped off, leaving the rooms exposed. The explosion set seven cars on fire and left a crater 15 feet wide.
The explosion occurred about 10:40 a.m., and the streets were full of people. It shattered windows at a courthouse more than half a mile away, sending shards flying that cut people who were attending a court hearing, said a lawyer who gave his name only as Abbas.
"How can you prevent these things from happening?" said the attorney as he sought treatment for his wounds. "We just have to establish an Iraqi government."
Abbas was one of more than 80 injured Iraqis who received treatment at the Nasiriyah hospital, the same facility where American prisoner of war Jessica Lynch was treated and later rescued at the height of the war.
The blast was the second-deadliest vehicle bombing since such attacks began in August. The worst was a car bombing at the Imam Ali Mosque in Najaf that killed at least 120 people in August.
The blast Wednesday killed 12 Italian Carabinieri, members of an elite military unit chosen to police the city. Four Italian soldiers, an Italian aid worker and an Italian filmmaker who was making a documentary about the deployment in Iraq also were among the dead. An additional 20 Italians were injured, authorities said.
Many of the Carabinieri were among the first of their unit to have arrived in Iraq four months ago and were scheduled to leave in a few days, officials said. The Italians apparently were well liked on the streets of Nasiriyah and were not perceived as being as tough as American troops in maintaining order.
The attack on international forces was prompting some countries to reevaluate their positions on troop deployments. Portugal said it was sending 128 elite police officers slated for Nasiriyah to Basra instead, the Associated Press reported. Japan said it was considering postponing its first dispatch of troops until sometime next year.