The worst day of violence since the handover of sovereignty exactly a month ago saw more than 100 people killed across Iraq.
Dozens of people were injured in the morning car bomb in Baquba.
Witnesses said a suicide bomber drove a vehicle into a crowded market area, as men queued to join the police.
White House spokesman Trent Duffy called the attack a "terrible tragedy... Our thoughts and prayers are with the families of the victims".
"This underscores why the Iraqi people, with the help of the world, must and will prevail over terrorism," he said.
Meanwhile, in Saudi Arabia, US Secretary of State Colin Powell was discussing whether Arab or Muslim states would be prepared to contribute troops to help keep stability in Iraq.
Saudi officials acknowledged talks were at an early stage, but said there was no agreement yet.
In Wednesday's violence:
seven Iraqi soldiers and 35 insurgents were killed in a joint multinational and Iraqi raid near the town of Suwariya, south of Baghdad
two soldiers serving with multinational forces were killed in clashes with insurgents in Anbar province, the US military said
a US soldier was killed and three others were wounded by a roadside bomb in north-west Baghdad, the US military said
eleven US soldiers were wounded and at least one Iraqi insurgent was killed in an attack on a US army camp outside Ramadi, west of the capital
at least one person was killed in a rocket explosion on a busy street in Baghdad
an Iraqi policeman was shot dead in the northern Iraqi city of Kirkuk
Gunmen stormed the home of the governor of Iraq's Al-Anbar province in Ramadi, kidnapping three of his sons
Burning wreckage
The Baquba attack was the worst since the 28 June transfer of power, and the bloodiest since a blast in the holy city of Najaf last August killed more than 80 people.
Among those killed in Wednesday's car bombing were 21 people travelling in a minibus, a health ministry official said.
I saw a car overtake a minibus and it slammed right into the queue of people," said Riad Abdul Latif, an internal affairs officer at the police station, who was 100m away when the bomb went off.
Police said young men had come to the police station to join the force. Because of the number of applicants, some had to queue outside.
After the blast, police put the dead and wounded in the back of pick-up trucks and drove them to hospital.
Men used hoses to douse the burning wreckage at the scene. Several bodies were also on fire amid the debris in what correspondents said were horrific scenes.
"God bless them, what have they done?" shouted one man of the victims.
Security forces have been a frequent target of attacks by groups opposed to the new government and US-led forces. Baquba, a mixed Sunni and Shia Muslim town, has experienced many attacks.
BBC News
Dozens of people were injured in the morning car bomb in Baquba.
Witnesses said a suicide bomber drove a vehicle into a crowded market area, as men queued to join the police.
White House spokesman Trent Duffy called the attack a "terrible tragedy... Our thoughts and prayers are with the families of the victims".
"This underscores why the Iraqi people, with the help of the world, must and will prevail over terrorism," he said.
Meanwhile, in Saudi Arabia, US Secretary of State Colin Powell was discussing whether Arab or Muslim states would be prepared to contribute troops to help keep stability in Iraq.
Saudi officials acknowledged talks were at an early stage, but said there was no agreement yet.
In Wednesday's violence:
seven Iraqi soldiers and 35 insurgents were killed in a joint multinational and Iraqi raid near the town of Suwariya, south of Baghdad
two soldiers serving with multinational forces were killed in clashes with insurgents in Anbar province, the US military said
a US soldier was killed and three others were wounded by a roadside bomb in north-west Baghdad, the US military said
eleven US soldiers were wounded and at least one Iraqi insurgent was killed in an attack on a US army camp outside Ramadi, west of the capital
at least one person was killed in a rocket explosion on a busy street in Baghdad
an Iraqi policeman was shot dead in the northern Iraqi city of Kirkuk
Gunmen stormed the home of the governor of Iraq's Al-Anbar province in Ramadi, kidnapping three of his sons
Burning wreckage
The Baquba attack was the worst since the 28 June transfer of power, and the bloodiest since a blast in the holy city of Najaf last August killed more than 80 people.
Among those killed in Wednesday's car bombing were 21 people travelling in a minibus, a health ministry official said.
I saw a car overtake a minibus and it slammed right into the queue of people," said Riad Abdul Latif, an internal affairs officer at the police station, who was 100m away when the bomb went off.
Police said young men had come to the police station to join the force. Because of the number of applicants, some had to queue outside.
After the blast, police put the dead and wounded in the back of pick-up trucks and drove them to hospital.
Men used hoses to douse the burning wreckage at the scene. Several bodies were also on fire amid the debris in what correspondents said were horrific scenes.
"God bless them, what have they done?" shouted one man of the victims.
Security forces have been a frequent target of attacks by groups opposed to the new government and US-led forces. Baquba, a mixed Sunni and Shia Muslim town, has experienced many attacks.
BBC News
