The death toll included seven police officers and four civilians.
“Today at around 8:40, in our neighborhood of Vezneciler, an attack by a car bomb on a bus carrying our police was carried out,” said Vasip Sahin, governor of Istanbul.
“As a result seven police (and) four civilians lost their lives. May God bless their souls. 36 people were injured. Three of them are in critical condition. Investigation is ongoing.”
Police established a security corridor and then widened the perimeter, urging people to vacate the area because a controlled explosion was to be carried out.
Local media reported that the blast, which occurred in the middle of morning rush hour, was remotely detonated.
Ambulances were on the scene and have taken the wounded to nearby hospitals, according to state news agency Anadolu.
Images from the blast area in the Beyazit Vezneciler district showed security personnel at the scene and visible damage to vehicle and buildings.
No group has taken responsibility for the bombing.
Erdogan: Target always human beings
Turkish President Recep Tayyip Erdogan offered his condolences and told journalists that he had visited the wounded.
“Let me be clear, terrorist organizations distinguishing between civilians, soldiers and police does not mean anything to us. The end target is always human beings,” he said.
“We will continue our fight against terrorists.”
France added its voice of condemnation, and President Francois Hollande expressed his “solidarity (with) the Turkish people.
“This intolerable act of violence must more than ever confirm our mutual determination to fight all forms of terrorism,” a statement read.
The U.S. Embassy in Turkey also expressed condolences via Twitter: “We condemn this heinous attack, and continue to stand shoulder-to-shoulder with Turkey in the fight against terrorism.”
Turkey has been rocked by multiple terror attacks this year. Attacks carried out by both the Kurdish militants and ISIS have targeted the capital, Ankara, and tourism hub Istanbul.
The last car bomb was in Ankara in March. That attack took place near a crowded bus stop, but militants who claimed the bombing said the intended target was nearby police officers.
Six days later an ISIS suicide bomber detonated himself on one of Istanbul’s main streets, killing three Israelis and one Iranian tourist.