At least 11 people have been killed and several wounded in a car bomb attack in central Istanbul.
A bus carrying Turkish police officers appeared to be the target of the blast, which struck during Tuesday morning's rush hour in the historic Beyazit Square district near Istanbul University and tourist sites.
Gunshots were heard in the area after the explosion, the Anadolu news agency reported, and emergency services were at the scene.
The governor of Istanbul, Vasip Şahin, said seven police officers and four civilians were killed, with 36 others wounded.
There was no immediate claim of responsibility. Isis, Kurdish militant groups and left-wing extremists have recently orchestrated attacks in Turkey.
A parked car packed with explosives was detonated using a remote control as the police bus passed, local reports said. The charred wreck of a patrially destroyed vehicle could be seen next to the bus, which was flipped upside down by the force of the explosion.
The blast also damaged nearby buildings and trees, leaving debris strewn across the road.
Attacks have mounted in Turkey as the Syrian civil war continues over the border and Turkish security forces continue operations in Kurdish areas of the country's south-east.
The country is part of the US-led coalition against Isis and allows international fighter jets to use an air base at Incirlik to launch raids.
Last month, two police officers were killed in a suspected Isis bombing at Gaziantep's police station and a suicide attack claimed by the Kurdistan Freedom Falcons (TAK) hit near a mosque in the city of Bursa in April.
An Isis suicide bomber targeted tourists on Istanbul's Istiklal Street on 19 March, six days after a TAK-claimed blast in central Ankara killed more than 30 people.
The violence has sparked several travel warnings by the US, UK and European nations cautioning that further attacks were likely.
The British Government's travel advice warns that attacks could be indiscriminate and "affect places visited by foreigners".
