Ten people are dead following a suicide bombing attack allegedly carried out by a Syrian in the Sultanahmet area of central Istanbul.
The Istanbul governor's office said 15 people were also injured in the blast, which struck at around 10:15 am (0815 GMT) near the Hagia Sophia museum and iconic Blue Mosque, both major tourist attractions on the European side of the metropolis.
Six Germans, one Norwegian and one Peruvian were among those injured, CNN Turk reported. The German Foreign Office said it was in touch with the Turkish authorities, but did not confirm the report.
Speaking in Ankara, President Recep Tayyip Erdogan said a person of Syrian origin is the suspected perpetrator of the bombing.
The president condemned the "terrorist" attack, but also used the speech to launch into a criticism of academics who have chastised Turkey's human rights policy in mostly Kurdish areas of the country, where there has been unrest for months.
Prime Minister Ahmet Davutoglu is set to convene a security meeting in Ankara later in the day.
A large group of police and emergency workers were at the scene after police cordoned off the area near the blast.
The government has also imposed a temporary broadcast ban in the wake of the explosion.
State television and other major stations were not showing images from the scene. Reporters in Sultanahmet, including a dpa journalist, were facing difficulties photographing and accessing the site.
Turkey - which borders both Iraq and Syria, two nations facing civil wars - has been facing increasing unrest during the past year.
The largest blast in the country's history took place in October, in the capital, Ankara. Two suicide bombers blew themselves up near a train station during a pro-Kurdish peace rally, killing 100 people.
Islamic State militants were blamed for that attack, as well as for another suicide blast months earlier in 2015, in the south of the country, which left more than 30 people dead. The group never claimed responsibility.
Meanwhile, violence in the south-east of the country has soared, as state security forces battle Kurdish militants after a peace process collapsed in the middle of last year.
Hundreds have died in the fighting, including members of the security forces, militants and civilians. Strict curfews have been imposed on some districts, drawing concern from human rights groups.
Istanbul has seen sporadic violence too, mostly from far-left groups. A mortar explosion at an airport in the city last month left one dead, with no clear claim of responsibility.