Eleven people including seven police officers were killed in a bomb attack targeting a central Istanbul district on Tuesday, the governor said.
"Seven police and four citizens lost their lives," Vasip Sahin told reporters at the scene. Thirty-six people were injured.
A parked car packed with explosives was detonated by remote control as the police bus drove by, CNN Turk said.
It broadcast cell-phone footage of a blackened and mangled vehicle on the street as vendors peered out from a souvenir shop in a district near Istanbul university.
There was no immediate claim of responsibility. But Kurdish militants, Islamic State and radical leftists have all staged attacks in Turkey recently.
A Reuters witness saw what appeared to be two police vehicles hit, one of them on its side next to the road. Gunshots were heard in the area after the blast, state-run Anadolu Agency reported.
Broadcasters showed armed police in the street near to the site where the blast struck. Haberturk said eight people were wounded.
A spokesman at police headquarters in Istanbul was unable to provide information on the incident when reached by phone.
Turkey has suffered a spate of bombings this year, including two suicide attacks in tourist areas of Istanbul blamed on Islamic State and two car bombings in the capital, Ankara, which were claimed by a Kurdish militant group