The largest European city which is not actually the capital of a country is Istanbul.
Istanbul is found in Euro-Asia, which means that it belongs both to Europe and Asia. It has a population of 14, 804, 116 and it is located in Turkey, although it is not its capital, but rather Ankara. The second largest non-capital city in Europe is Saint Petersburg in Russia, with the population of 5, 323, 300 people.
