The force of the blast caused the police bus to overturn
A car bomb attack on a police bus in Istanbul has killed at least 11 people, seven of them officers.
"Seven police and four citizens lost their lives," the city's governor, Vasip Sahin, said at the scene in the Beyazit district.
Thirty-six others were injured. At least three of them were in a serious condition.
The explosion took place close to the Vezneciler metro station, which is within walking distance of some of the city's main historical sights, including the famed Suleymaniye Mosque.
It was also near an Istanbul University building, forcing officials to cancel exams.
The metro station was closed as a security precaution.
The force of the blast caused the police bus - carrying rapid-response officers - to overturn.
Nearby buildings were damaged too, including a hotel which had its windows blown out. Several cars were wrecked.
"There was a loud bang, we thought it was lightning but right at that second the windows of the shop came down," said a local shopkeeper, Cevher.
Goods toppled from his shelves.
"It was extremely scary," he said.
It is the fourth major bombing in Turkey's biggest city this year - two targeting security forces and the other two targeting tourists.
The violence has led to a sharp dip in tourism, damaging the economy.
Mr Sahin did not comment on who may be behind the attack, but the Kurdistan Workers' Party, or PKK, has been targeting police and military personnel with bombs since July, after a fragile peace process between it and the government collapsed.
Islamic State has also been blamed for a series of bombings in Turkey.
In a TV interview, foreign minister Mevlut Cavusoglu condemned the attack, saying: "They are cold-heartedly exploding bombs on a Ramadan day."