A bomb attack targeted Turkish police in a central Istanbul district on Tuesday, leaving several people killed

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, including one last month in Istanbul.

"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.

Three of the 36 wounded were in critical condition, he added.

The remote-controlled bomb exploded as a service shuttle carrying police officers was passing in the Beyazit district of Istanbul, the state-run TRT television reported.

Ambulances and fire engines were dispatched to the scene. Five people were injured according to initial findings, the Dogan news agency reported.

Reports said the explosion took place close to the Vezeciler metro station, which is within walking distance of some of the main sights of the historical center including the Suleymaniye Mosque.

Since the start of the year, Turkey has been hit by a sequence of attacks that have rattled citizens and also caused tourism to plummet.