Initial reports from city authorities in Istanbul said that 10 people had been killed and 15 injured in the blast in the Sultanahmet area of the city, one of the busiest destinations for tourists thanks to its many mosques and other cultural sites.
The Norwegian Ministry of Foreign Affairs told news agency NTB that it one Norwegian was injured in the blast.
"We can confirm that one Norwegian was slightly injured and that he is being treated at a Turkish hospital," ministry spokeswoman Ane Lunde said.
Video from the scene showed large numbers of emergency vehicles arriving, while photos showed bloodied bodies lying on the ground.
Al Jazeera reported that an eyewitness said the attack was a suicide bombing. Turkish officials said there was a link to terrorism.
The blast was large enough to be heard up to a mile away.
Turkish journalists tweeted soon after the blasts that Germans were among the victims, although there were no figures available immediately.
CNN Turk reported that tourists from Germany, Norway, and East Asia were among those wounded.
Citing CNN Turk, Germany's 24-hour news broadcaster n24 reported that six Germans had been wounded.