Ten people are dead following a suspected suicide bomb attack in the popular touristic Sultanahmet area of central Istanbul.
The Istanbul governor's office said 15 people were also injured. The blast occurred at around 10:15 am (0815 GMT).
Broadcaster CNN Turk cited police sources saying they suspect a suicide bomber launched the attack, which struck near the Hagia Sophia museum and Blue Mosque, both major tourist attractions on the European side of the metropolis.
The broadcaster reported six Germans were among those injured, along with a Norwegian and a Peruvian. The German Foreign Office said it was in touch with the Turkish authorities, but did not confirm the report.
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.
Omer Celik, a spokesman for the governing Justice and Development Party (AKP), condemned the "vile attack," in a post on his Twitter feed.
Prime Minister Ahmet Davutoglu is set to convene a security meeting in Ankara later in the day.
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.
The Islamic State was 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.
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.