They said they expected the death toll in Thursday's simultaneous blasts to rise.
"As the police patrol was passing by, a car bomb exploded. At the same time a suicide bomber went inside the shop and blew himself up," a policeman at the scene told Reuters.
He said at least 12 civilians and three policemen had been killed in the explosions.
In the aftermath of the blasts, body parts lay scattered in the street and several cars were on fire.
The bombings took place just hours after Iraqi security forces launched a major clampdown in the capital, in advance of Friday's release of the final results of the 15 December elections.