At least 55 people have been killed and hundreds injured in a wave of bombings and shootings across Iraq, police say.The violence targeted predominantly Shia areas, in particular police officers and checkpoints.Dozens were killed in Baghdad, with attacks targeting commuters and crowds. One car bomb in the upmarket Karrada district killed nine people.No group has yet said it was behind the violence. Attacks in Iraq have risen since US troops withdrew in December.Tolls from other attacks around Baghdad include:Six dead after a car bomb in Shia-dominated Kadhimiya, north of BaghdadSix killed by gunmen at a police checkpoint in the Sarafiya district of the capitalTwo dead and five injured in an explosion in the western al-Mansour districtTwo killed and 10 injured in two explosions in Dorat Abo Sheer, southern BaghdadTwo killed and nine wounded in an attack by gunmen using weapons with silencers, targeting a police patrol in Saidiya, southern BaghdadSeven injured, most of them policemen, in a blast in al-Madaen, south of BaghdadFive civilians injured in a bomb explosion in Taji, north of BaghdadThere were also attacks in Mosul, Kirkuk and the province of Salahuddin.