Police officials say the bomb went off after night prayers today as the worshippers were leaving al-Hasanien mosque in the primarily Sunni neighbourhood of Dora.
A hospital official confirmed the casualties. All officials spoke on condition of anonymity because they are not authorised to speak to reporters.
There was no immediate claim of responsibility.
Al-Qaeda's Iraqi branch, known as the Islamic State of Iraq, often attacks civilian targets, including mosques. It primarily targets Shiites as well as security forces. In the past it has also struck Sunni targets in an attempt to reignite sectarian fighting.