"A suicide bomber blew up his explosive belt while he was surrounded by police near a public garden in Al-Shoala district," a mainly Shiite northern neighbourhood of the capital, a security forces statement said.
A police source told AFP four people had been killed and 16 others wounded in the overnight bombing.
Those killed were a woman, a girl and three police officers, according to the source.
Images from the scene circulated on social media showed police vehicles with their bodywork damaged and windows blown out.
Cafes in Iraq are particularly busy at night during the Islamic holy month of Ramadan, with families and friends gathering to relax and eat before the fast re-starts at dawn.
Iraq in December claimed victory over the Islamic State group after a gruelling campaign to retake swathes of territory captured by the militants, but IS still carries out sporadic bombings.