At least two people have been killed and eight reportedly injured by a blast which targeted a police bus in central Istanbul, Turkish media say.
While no group has yet taken responsibility of the attack, the remote-controlled device was reportedly detonated as the vehicle passed through the busy Vezneciler district during the morning rush hour, reported BBC News.
Violence in Turkey has escalated recently as a result of tensions with Kurdish separatists and the conflict in neighbouring Syria.
The explosion happened near the city's historic Beyazit Square neighbourhood, a major tourist attraction.
Pictures showed the wreckage of a bus destroyed and the facade of nearby buildings damaged. Armed police were also seen next to the site.
According to reports, gunfire was heard in the area after the blast.
Turkey is on high security alert after two deadly attacks in Istanbul this year blamed on Islamic State of Iraq and the Levant (ISIL) group, and a pair of attacks in Ankara that were claimed by Kurdish separatists and killed dozens, reported Al Jazeera.
The two attacks in Ankara were claimed by the Kurdistan Freedom Falcons (TAK) - a splinter group of the better-known outlawed Kurdistan Workers' Party (PKK).
President Recep Tayyip Erdogan has said terror groups are targeting civilians because they are losing their struggle against Turkish security forces.
Turkey is part of the US-led coalition against IS and allows coalition planes to use its air base at Incirlik for raids on Iraq and Syria, reported BBC News.
A two-year-old ceasefire between Turkey and the Kurdish militant PKK broke down last summer.