An explosion in Istanbul's prominent tourist area killed at least ten people and wounded 15 others, according to Istanbul governor's office.
The blast occurred at the Sultanahmet district in central Istanbul, near the Hagia Sophia and the Blue Mosque.
No group immediately claimed responsibility.
The blast comes at a time when Turkey is dealing with multiple threats to its security.
Sultanahmet district of Istanbul
The Sultanahmet district is the heart of historic Old Istanbul, what 19th-century travelers used to call "Stamboul." The area is Istanbul's tourist hub with attractions like Topkapi Palace, Ayasofya (Hagia Sophia), the Blue Mosque (Sultan Ahmet Camii), the Byzantine Hippodrome, etc.
German, Norwegian tourists among those wounded in blast
Turkey's Dogan news agency said that at least six Germans, one Norwegian and one Peruvian were among the injured.
Germany's foreign office issued a travel advisory, following the blast.
Germany urged its citizens to avoid crowds outside tourist attractions in Istanbul.
Germany's Foreign Ministry also warned on its website that further violent clashes and "terrorist attacks" are expected across Turkey.
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Terrorist links suspected
Turkey's Prime Minister Ahmet Davutoglu convened an emergency security meeting of key ministers and officials.
As with previous attacks, authorities imposed a news blackout, barring media from showing images of the dead or injured or reporting any details of the investigation.
A government official said the explosion was believed to be "terror-linked".
Some local reports suggest that blast was caused by a suicide bomber.
Recent blasts in Turkey
In October, two suicide blasts in the capital Ankara killed more than 100 people and more than 30 people were killed in an attack in the town of Suruc, near the border with Syria in July 2015.
President Erdogan and European leaders condemn the attack
Turkish president Tayyip Erdogan said, "I condemn the terror incident in Istanbul assessed to be an attack by a suicide bomber with Syrian origin."
Donald Tusk, president of the European Council, described it as "brutal terrorist attack".
Speaking on behalf of EU, foreign affairs spokeswoman Federica Mogherini and commissioner Johannes Hahn said that, "the EU extends sincere condolences to the families of the victims."
Blast carried out by ISIS: Ahmet Davutoğlu
Turkish authorities have attributed the Istanbul bombing to a member of ISIS from Syria.
Turkish Prime Minister Ahmet Davutoğlu stated that, "We have determined that the perpetrator of the attack is a foreigner who is a member of Daesh."
The 28-year old Syrian, believed to be the bomber, allegedly entered Turkey from Syria but was not on a watch list of suspected militants.