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 hit security targets.
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.
Istanbul's governor, Vasip Sahin, said about 36 other people were injured.
Turkey in midst of hideous vortex
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."
Turkey's Foreign Minister, Mevlut Cavusoglu, said the attackers were "cold-heartedly" for exploding bombs on the holy Muslim month of Ramadan, which is in its second day.
An eyewitness told Reuters news agency that there was a "loud bang", strong enough to topple all the goods from the shelves of his store.
"We thought it was lightning, but right at that second the windows of the shop came down. It was extremely scary," he said.
Recent attacks in Turkey
- March 2016: 35 people killed by Kurdish militants in Ankara; 4 dead in a suicide attack in Istanbul
- February 2016: 28 killed in military convoy in Ankara
- January 2016: 12 German tourists are killed in a suspected IS suicide bombing in Istanbul
- October 2015: More than 100 people are killed in a double suicide bombing at a Kurdish peace rally in Ankara
The Kurdistan Workers' Party (PKK)
Tears and destruction amid PKK crackdown
Turkey has been hit by bloody attacks in recent months by the so-called Islamic State (IS) group and Kurdish militants.
Mr Erdogan has said militants are targeting civilians because they are losing their struggle against Turkish security forces.
Turkey is part of the US-led coalition against IS and allows coalition planes to use its air base at Incirlik for raids on Iraq and Syria.
The country has also been waging an offensive against the PKK, which fights for autonomy in the majority-Kurdish areas in Turkey's south-east.
A two-year-old ceasefire between Turkey and the PKK broke down last summer. Since then, violence has killed hundreds of Turkish security forces, Kurdish fighters and civilians.
Turkey, the EU and US refer to the PKK as a terrorist organisation.
An offshoot of the group, the TAK, carried out two attacks on police vehicles in Ankara this year.
The government and western intelligence groups believe TAK is an alias of the PKK.
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