Investigators said the bomb, believed to contain more than 6 pounds of explosive, was concealed in a plastic bag, CNN reported. It went off at 10 p.m. in the coffee shop, on the third floor of a building in the al-Amriya neighborhood that also holds an ice cream parlor and doctors' offices.
More than 50 people were injured. Most of the victims were young men.
Iraqis are scheduled to vote Saturday in provincial elections, which have been preceded by a wave of violence. Martin Kobler, the U.N. secretary-general's special representative for Iraq, urged increased security so Iraqis can go to the polls safely.
"I am calling on all women and men to cast their ballots for a better future for them and their children," Kobler said. "My appeal goes particularly to young Iraqis, because you are the future of this country."