The blast sent panicked shoppers scurrying into side alleys off Istiklal Street, a long pedestrian avenue lined with international stores and foreign consulates, a few hundred metres from where police buses are often stationed.
The attack will raise further questions about NATO member Turkeys ability to protect itself against a spillover of violence from the war in neighbouring Syria.
Turkey faces threats from Kurdish militants, whose insurgency has spread from the largely Kurdish southeast and who Ankara sees as closely linked to a Kurdish militia in Syria, and from Islamic State fighters, who have also recently targeted it.
Germany shut down its diplomatic missions and schools on Thursday, citing a specific threat. Meanwhile, U.S. and other European embassies had warned their citizens to be vigilant ahead of Newroz celebrations this weekend, a spring festival largely marked by Kurds which has turned violent in the past.
There was no immediate claim of responsibility for the bombing, which two senior officials said could have been carried out by the Kurdistan Workers Party (PKK), fighting for Kurdish autonomy in the southeast, or by an Islamic State militant.
Both groups have targeted Turkey in recent months. A PKK offshoot claimed responsibility for two suicide bombings in the capital Ankara over the past month, which killed a total of 66 people. And Islamic State was blamed for a suicide bombing in Istanbul in January which killed at least 12 German tourists.
One of the officials said the bomber had planned to hit a more crowded location but was deterred by the police presence.
"The attacker detonated the bomb before reaching the target point because they were scared of the police," the official said, declining to be named as the investigation is ongoing.
Another official said investigations were focusing on three possible suspects, all of them male and two of them from the southern city of Gaziantep near the Syrian border. There was no further confirmation of this.
Armed police sealed off the shopping street where half a dozen ambulances had gathered. Forensic teams in white suits searched for evidence as police helicopters buzzed overhead.
"I saw a body on the street. No one was treating him but then I saw someone who appeared to be a regular citizen trying to do something to the body. That was enough for me and I turned and went back," one resident told Reuters.
Istiklal Street, usually thronged with shoppers at weekends, was quieter than normal as more people are staying home after a series of deadly bombings.
Health Minister Mehmet Muezzinoglu confirmed that 36 people had been wounded, seven of those were in serious condition and twelve of the wounded were foreigners.
Broadcaster NTV said six of the wounded were Israeli tourists and two others were from Iceland. Israels foreign ministry confirmed some of its citizens were among the wounded.
International condemnation
Saturdays blast came as Turkey is still in shock from a suicide car bombing on Sunday at a crowded transport hub in the capital Ankara which killed 37 people. A similar bombing in Ankara last month killed 29. An offshoot of the PKK claimed responsibility for both bombings.
The Istanbul suicide bombing which killed German tourists in January struck at its historic heart and was blamed by the government on Islamic State.
Saturdays attack brought condemnation from around the globe.
Iranian Foreign Minister Mohammad Javad Zarif, on an official visit to Istanbul, said it showed "the ugly face of terrorism", while France condemned it as "despicable and cowardly".
Germany urged tourists in Istanbul to stay in their hotels.
Turkeys Peoples Democratic Party (HDP), a Kurdish-rooted opposition party, condemned the bombing.
"Just as in the Ankara attack, this is a terrorist act that directly targets civilians," the HDP said in an e-mail. "Whoever carried out this attack, it is unacceptable and inexcusable."
Turkey is a member of the U.S.-led coalition fighting Islamic State in Syria and Iraq. It is also battling the PKK in its own southeast, where a 2-1/2-year ceasefire collapsed last July, triggering the worst violence since the 1990s.
In its armed campaign in Turkey, the PKK has historically struck directly at the security forces and says it does not target civilians. However, recent bombings suggest it could be shifting tactics.
One of the Turkish officials said the PKK was looking to carry out attacks during the Newroz holiday.
At the height of the PKK insurgency in the 1990s, the festival was often marked by violent clashes between Kurdish protesters and security forces. It coincides with the spring thaw, a time when in previous years PKK fighters re-entered Turkey from mountain hideouts in northern Iraq.(*)
Editor: Heru Purwanto
Copyright © ANTARA 2016
