No organization has claimed responsibility, officials said, though the city has been a target in the past of Kurdish separatist militants and Al-Qaeda, as well as militants from Turkey's far-left.

Istanbul police chief Huseyin Capkin said a man had approached the police stationed at the square before blowing himself up. Television footage taken immediately after the explosion showed police firing warning shots and people fleeing in panic.

Fifteen policemen and 17 civilians were wounded in the attack at 10:40 a.m. in Taksim Square, but only nine, mostly policemen, were kept in hospital, Istanbul Gov. Huseyin Avni Mutlu told reporters.

Taksim Square is a tourist and transport hub surrounded by restaurants, shops and hotels at the heart of Istanbul.

The bomber struck near police buses parked close to a monument commemorating Mustafa Kemal Ataturk, the founder of modern Turkey, and victory in the war of independence in 1923.

Mobile phone footage taken just after the explosion showed a woman lying close to the monument bleeding heavily from her leg, and a policeman also lying with blood streaming from his head.

A taxi driver told CNN Turk news channel he saw a man in his early thirties approach the police to ask directions, at which point the bomb detonated. Another witness said he saw two men.

According to the governor, police seized plastic explosives with a detonator at the scene, though it was unclear whether they had been part of a second bomb.

The city has been targeted before by Kurdistan Workers Party rebels, but the separatist group extended a unilateral cease-fire last month, and on Saturday said it would announce in the coming days whether it would extend it further.