A car bomb ripped through a police bus in central Istanbul during the morning rush hour on Tuesday, killing 11 people and wounding 36 near the main tourist district, a major university and the mayor's office.
The car was detonated as police buses passed, Istanbul Governor Vasip Sahin told reporters, in the fourth major bombing in Turkey's biggest city this year.
There was no immediate claim of responsibility but Kurdish militants have staged similar attacks before, including one last month in Istanbul.
Security concerns were already hitting tourism and investor confidence. Wars in neighbouring Syria and Iraq have fostered a home-grown Islamic State network blamed for a series of suicide bombings, while militants from the largely Kurdish southeast have increasingly struck in cities further afield.
President Tayyip Erdogan visited the wounded in a nearby hospital. Sources in his office said he had condemned the attack and had been briefed by Sahin and the interior minister.
"A car-bomb attack was made against vehicles carrying our rapid-response police and passing by on the road, resulting in seven police and four civilians losing their lives," Sahin said, adding three of the 36 wounded were in critical condition.
The blast hit the Vezneciler district, between the headquarters of the local municipality and the campus of Istanbul University, not far from the city's historic heart. It shattered windows in shops and a mosque and scattered debris over nearby streets.
"There was a loud bang, we thought it was lightning but right at that second the windows of the shop came down. It was extremely scary," said Cevher, a shopkeeper who declined to give his surname. The blast was strong enough to topple all the goods from the shelves of his store.
The police bus that appeared to have borne the brunt of the explosion was tipped onto its roof on the side of the road. A second police bus was also damaged. The charred wreckage of several other vehicles lined the street.