Insurgents lured Iraqi policemen to a house in west Baghdad and set off a huge amount of explosives, killing at least 29 people, seven of them police.
The blast also flattened surrounding residential buildings
The ambush happened in the ramshackle Ghazalia district after police received a tip-off about a militants' hideout.
The blast was set off by remote control as police were inside. The building and several other houses were flattened.
The incident, late on Tuesday, ended a day of unrelenting violence in Sunni Muslim areas north of Baghdad.
Dozens of police were killed in a string of apparently coordinated attacks in Tikrit, Samarra, Baquba, and in Baghdad itself where a National Guard general narrowly escaped a car bomb outside his home.
Several people were believed to be trapped under the rubble after Tuesday's blast, and four policemen are reported missing.
The police said they responded to a call from a neighbour saying that there was shooting coming from a house.
"When the police arrived and went in, the house blew up. It seems to have been a trap, " one police officer said
"The house was turned into a bomb."
Civilians seem to have borne the brunt of the blast
The US military said that American soldiers and Iraqi troops worked through the night in the search for survivors.
Initial findings indicated that 700-800kg (1,700-1,800lb) of explosives had been used in the attack, the US military said.
The BBC's Jim Muir in Baghdad says Tuesday's attacks demonstrated that insurgents were not only well organised but capable of hitting a range of targets.
In one of the attacks, a police station in Dijla was stormed by gunmen who executed 12 officers by slitting their throats, according to one report.
Brig Gen Geoffrey Hammond, a senior officer in the US army brigade that controls Baghdad, said he expected insurgents would continue their efforts "to destroy life in Baghdad".
Violence has increased in the run-up to US-backed national elections set for 30 January.