At least 10 people have died and other 15 injured in an explosion in Istanbul’s Sultanahmet district near the Blue Mosque, the city authorities say.
Foreign tourists are reportedly among those hurt in the blast.
Some reports suggest the explosion was caused by a suicide bomber.
Norway’s foreign ministry said a Norwegian man injured in the blast was receiving treatment in hospital, Reuters reported.
Germany’s foreign ministry is investigating reports that German citizens have been hurt in the blast, German media reported.
The ministry has on its website urged German tourists in the city to avoid large crowds and tourist attractions and warned that further violent clashes and “terrorist attacks” are expected across Turkey.
In recent months there have been sporadic attacks by a far-left group in Istanbul, while violence has soared between Turkish forces and PKK Kurdish militants, mainly in southeast Turkey, after a ceasefire broke down.
Turkey has also been hit by bomb attacks blamed by prosecutors on ISIS.
Two suicide blasts in the capital Ankara in October killed more than 100 people and more than 30 people were killed in an attack near the border with Syria in July.