Turkey’s president Tayyip Erdogan has said a Syrian suicide bomber is thought to have been responsible for Tuesday’s explosion in Istanbul’s historic district which killed 10 people.
"I condemn the terror incident in Istanbul assessed to be an attack by a suicide bomber with Syrian origin. Unfortunately we have 10 dead including foreigners and Turkish nationals ... There are also 15 wounded," Erdogan said, in a live broadcast from Ankara.
Several bodies lay on the ground in the Sultanahmet square, close to the Blue Mosque and Hagia Sophia, a major tourist area of Turkey's most populous city.
A police officer and witness at the scene reported also seeing several bodies and body parts.
Six German citizens, one Norwegian and one Peruvian were among the wounded, the Dogan news agency said.
A tour company official told Reuters a group from Germany was in the area at the time but said there was no immediate information on whether any of them had been injured.
An official at the German foreign ministry said it could not be ruled out that German citizens may have been injured and that its crisis unit and the consulate in Istanbul were urgently working with the Turkish authorities to find out.
Norway's foreign ministry said one Norwegian man was injured and was being treated in hospital.
The attack comes as Turkey battles Kurdish militants in its southeast and Islamic State insurgents just across its southern borders in Syria and Iraq.
There was no immediate claim of responsibility but Islamist, leftist and Kurdish militants have all carried out attacks in Turkey in the past.
The Istanbul governor's office said the authorities were investigating the type of explosive used and who might have been responsible. It said 10 people were killed and 15 wounded but gave no further details.
"We heard a loud sound and I looked at the sky to see if it was raining because I thought it was thunder but the sky was clear," said Kuwaiti tourist Farah Zamani, 24, who was shopping at one of the covered bazaars with her father and sister.
A police officer at the scene said the square was not densely packed at the time of the blast, but that small groups of tourists were wandering around.
"It was unimaginable," he said, describing an amateur video he had seen of the immediate aftermath, with six or seven bodies lying on the ground and other people seriously wounded.
Turkey's A-Haber television said the blast may have been caused by a suicide bomber but this was not independently confirmed. Ambulances rushed to the scene, ferrying away the wounded as police cordoned off streets, fearing a second attack.
Prime Minister Ahmet Davutoglu held an emergency meeting in Ankara with the interior minister and security chiefs. A senior official said "terrorist links" were suspected in the attack, but declined to comment further.
The dull thud of the explosion was heard in districts of Istanbul several kilometres away, residents said. Tourist sites including the Hagia Sophia and the nearby Basilica Cistern were closed on the governor's orders, officials said.
The sound of the call to prayer rang out from the Blue Mosque as forensic police officers worked at the scene.
Just over a year ago, a female suicide bomber blew herself up at a police station for tourists off the same square, killing one officer and wounding another. That attack was initially claimed by a far-left group, but officials later said it had been carried out by a woman with suspected Islamist militant links.
Turkey has become a target for Islamic State, with two bombings last year blamed on the radical Sunni Muslim group, in the town of Suruc near the Syrian border and in the capital Ankara, the latter killing more than 100 people.
Davutoglu's office imposed a broadcasting ban on the blast, invoking a law which allows for such steps when there is the potential for serious harm to national security or public order.