The Tawhid and Jihad group said in an unverified statement, "With the grace of God, a lion from our martyrdom brigades was successful in striking a center for apostate police volunteers."
The group also claimed responsibility for a separate gun attack that killed 12 policemen when gunmen opened fire on their van in Baqouba.
"The heroes of the Tawhid and Jihad group struck a car in which there were more than 10 agents of the police, leading to their deaths," said the statement.
The United States has offered a $25 million reward for the capture of al-Zarqawi -- its top target in Iraq, Reuters reported.
In the attack near police headquarters, hundreds of police recruits were lined up outside when the car bomb loaded with artillery shells exploded, burning nine cars and grinding a 10-foot crater into Haifa Street.
Many of the 47 dead were people shopping nearby and eating at cafes, according to the Associated Press.
At least 14 people were wounded, Health Ministry spokesman Saad Al-Amili said. The bomb was in a Toyota parked near the shops and the police headquarters, which was closed to traffic.
"It was a horrific scene. Seconds earlier people were drinking tea or eating sandwiches and then I could see their remains hanging from trees," Mahdi Mohammed, who was outside his barbershop at the time of the explosion, told the Associated Press. "I could see burning people running in all directions."
Crowds of young men chanted "Bush is a dog," saying that President Bush and interim Iraqi Prime Minister Iyad Allawi had failed to protect Iraqis.
Other people, however, expressed anger at militants.
"Such acts cannot be considered part of the resistance (against American forces). This is not a jihad, they are not mujahideen," Amir Abdel Hassan told the AP. "Iraq is not a country, it's a big graveyard," Hassan said.
Gunmen in two cars fired at the policemen in a van returning from a training session that had been postponed, the AP reported.
This was not the first attack on security forces in the area. On July 28 a car bomb exploded in Baqouba outside a police recruiting center, killing at least 68 people.
Two American soldiers and three others were wounded in Baghdad when they were attacked by an explosive device and small arms fire on Monday, according to U.S. officials.
On Tuesday a Task Force Olympia soldier was killed and five were injured when they came under attack by small arms fire in the northeastern city Mosul.