The remote-controlled bomb exploded as a service shuttle carrying police officers was passing in the Beyazit district of Istanbul, 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.
Turkey is on high security alert after two deadly attacks in Istanbul this year blamed on Islamic State (IS) jihadists, and a pair of attacks in Ankara that were claimed by Kurdish militants and killed dozens.
The two attacks in Ankara were claimed by the Kurdistan Freedom Falcons (TAK) - a radical splinter group of the better-known outlawed Kurdistan Workers' Party (PKK).