Capkin said police had been the intended target. He did not say who was suspected of responsibility, but the separatist rebel Kurdistan Workers' Party (PKK) and extreme left-wing groups have carried out bomb attacks in the past in Istanbul, home to more than 12 million inhabitants.

The blast happened in Taksim Square, a major shopping and leisure district on the European side of the city.

The busy square and the roads leading to it attract tens of thousands of people a day and are patrolled by anti-riot police around the clock.

According to witnesses quoted by news channels, the powerful blast shook the area around 10.30 am (0830 GMT).

Footage showed police and ambulances rushing to the scene.

Police immediately threw a security cordon around the area, and streets including the pedestrianised historic Istiklal were closed off, the CNN-Turk and NTV channels reported.

On June 22 a bomb exploded in a suburb of Istanbul as a bus full of soldiers was going past, killing five soldiers and the daughter of one of them.

Responsibility was claimed by the Kurdistan Freedom Falcons (TAK), an obscure radical group loyal to jailed PKK leader Abdullah Ocalan, jailed for life in 1999.

The Turkish authorities say TAK is a front used by the PKK, especially when attacks claim civilian casualties.

The PKK has said TAK is a splinter group outside its control.

Murat Karayilan, a top commander of the outlawed PKK, last week said the PKK would no longer target civilians and wanted to extend a unilateral truce indefinitely if Turkey demonstrates a commitment to dialogue.

"We are actually in favour of a permanent ceasefire... We are waiting. We have not decided yet," he told the Radikal newspaper.

The PKK, which has waged a bloody 26-year campaign for self-rule in mainly Kurdish southeast Turkey, announced a unilateral truce on August 13, due to expire at the end of October.