The car was parked near a restaurant in Tikrit, about 175km north of the capital, Baghdad. The blast set nearly a dozen vehicles on fire, police sources said on Sunday.
Three of the dead were women, sources told the AFP news agency. Tikrit's main hospital said it had received several patients wounded in the attack.
There were no immediate claims for the attack, though the Islamic State (also know as IS, ISIS) has claimed responsibility for several recent attacks in Iraq.
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