A car bomb attack targeting a police bus has killed seven officers and four civilians in central Istanbul.
The explosives were remotely detonated as the vehicle passed through the busy Vezneciler district at the morning rush hour, officials said.
No group has said it carried out the attack, but Kurdish militants have recently targeted security services.
President Recep Tayyip Erdogan says Turkey will "fight the terrorists to the end".
The explosion happened near the city's historic Beyazit Square neighbourhood, a major tourist attraction, and an Istanbul university building.
About 36 other people were injured, Istanbul's governor, Vasip Sahin, said.
REUTERS
SNS
Visiting some of the wounded in hospital, Mr Erdogan said: "These (attacks) are being carried out against people whose duty it is to ensure the security of our people.
"These cannot be pardoned or forgiven. We shall continue our fight against terrorists tirelessly until the end."
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