It's the fourth major attack in Turkey's largest city this year and while no group has yet claimed responsibility, Kurdish militants have been responsible for similar attacks.
During the morning peak in a major tourist district in Istanbul, a bomb was detonated remotely just as a police bus was passing by.
Eyewitness Ridvan Alp says those responsible had packed a car full of explosives.
"We heard the sound of an explosion. They exploded the bomb when the police bus was passing by. It was a car laden with bombs. The car was waiting right there. They detonated it while the police bus was driving past. Ambulances arrived at site immediately. The police were already here."
The bomb exploded in front of a hotel and its force shattered windows in nearby buildings.
Seven police officers and four civilians died and 36 others were injured.
The banned Kurdistan Workers Party, the PKK, has claimed responsibility for previous attacks.
The PKK is fighting for autonomy in the majority-Kurdish parts of Turkey's south-east and attacks are now regular occurrences since the breakdown of a ceasefire between Turkey and the PKK.
Turkey's President Recep Tayyip Erdogan, has condemned the bombing.
"These terrorist attacks are being carried out against those responsible for providing security. This is unforgivable, inexcusable. We will continue our struggle against the terrorists until the end, fearlessly and tirelessly."
He visited the wounded in hospital where one victim told him this was the second time she'd been caught up in a terror attack.
"In a previous attack, the suicide bomber blew himself up in front of (my) eyes. This is the second time I am having this ordeal."
Another of the wounded told the President he was standing in front of the hotel that took the force of the blast.
"I was in front of the hotel. I heard the sound of explosion. I had two incoming messages on my mobile and I was being impatient to read them. As I tried to read them, I heard the explosion."
The self-proclaimed Islamic State has also carried out attacks against Turkey, which is part of the United States-led coalition against I-S.
And political analyst Mehmet Yegin has told the ABC Turkey is not just in conflict with the PKK and IS.
"People are asking why are we fighting with so many terrorist organisations together and while Turkey declared fight with both PKK and ISIS, PKK also made certain alliances with other Stalinist terrorist organistions so we are now fighting nearly 10 terrorist organisations together."
Share
