The Iraqi militant group Ansar al-Sunnah claimed responsibility for the attack in the Kurdish city of Irbil, saying in an internet statement it was in revenge for the Kurds' alliance with US forces.
The same group had claimed credit for a 2004 twin suicide bombing also targeting Kurds in Irbil that killed 109 people.
At least seven cars parked nearby were destroyed by the blast in an upscale residential neighbourhood that includes a Sheraton Hotel. Several nearby buildings were damaged in the city, which is 217 miles north of Baghdad. One hospital became so crowded that staff members used a loudspeaker to give the names and room numbers of the victims to relatives who rushed there.
At least 46 people were killed and 100 wounded, said Gov Nozad Hadi and Dr Burhan Saleh of Irbil Teaching Hospital, where some of the casualties were treated.
US Navy chief petty officer James Drake confirmed those figures, but said that there were conflicting reports about whether the blast was caused by a pedestrian carrying hidden explosives or a car bomb. The claim of responsibility by Ansar al-Sunnah said its attack involved a suicide car bomb.
In Irbil, police Capt. Othman Aziz said that an Iraqi man standing among dozens of recruits who were in line outside the two-story building where every entrant is searched by guards. Shortly before reaching the entrance, the attacker detonated himself, Aziz said.
Iraqi civilian Hawra Mohammed, 37, said he had just dropped his brother Ahmed, 32, off at the centre to apply for a job and driven away when the explosion occurred.
When Hawra raced back, he found his brother lying in a street, bleeding and unconscious. But Ahmed soon began to move.
"I lifted my brother onto my shoulders and took him to a nearby hospital," Hawra said in an interview. "The blood on my shirt is my brother's."
The attack appeared to be the deadliest by insurgents in Iraq since February 28, when a suicide car bomber struck a crowd of police and national guard recruits outside a medical clinic in Hillah, south of the capital. That attack, which killed 125 people and wounded more than 140, was the deadliest in the insurgency.
Militants have stepped up their attacks across Iraq in the last week, often targeting convoys of US and Iraqi troops, and Iraqi police on patrol or at recruitment centres.
A key goal of US troops is to train enough Iraqi security forces to reduce the role being played by Americans in fighting the insurgency.
Elsewhere, the US military said two American soldiers were killed in separate roadside bomb attacks in Baghdad the day before. Those explosions, and the Irbil blast, raised the death toll from a week of insurgent-related violence in Iraq to nearly 200.
As of Tuesday, at least 1,585 members of the US military have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.
In other developments yesterday:
The Iraqi government said its security forces captured Ayman Sabawi, a son of one of Saddam Hussein's half brothers, in a recent raid on suspected insurgents.
A car bomb exploded south of Baghdad in Hillah near the offices of the Shi'ite Dawa party, police said. Nearby vehicles and buildings were damaged, but no casualties were reported.
An Australian taskforce arrived in Baghdad to work for the release of Douglas Wood, a kidnapped Australian citizen and US resident.