Iraqi officials said a suicide attacker has detonated an explosives-packed car at a funeral in a Shi'ite area of Baghdad, killing at least 20 people, the Associated Press reported.
Police said the attacker struck today in the Iraqi capital's Sadr City neighbourhood.
At least 40 people have been injured, police told the news agency.
The attack followed a suicide assault on a police headquarters and other insurgent attacks in northern Iraq earlier in the day that killed 11 members of the security forces.
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Authorities have warned that the death toll is likely to rise as much of the building is yet to be searched.
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In the unexpected announcement on Monday, Mr Key said it was the "right time" to leave politics.