Greece is divided into 52 prefectures. Each one has a capital which could be considered a city. It does not however, mean that all cities are heavily populated or demographically large.
Athens is the capital
The top 6 by population:
Athens (3,361,806), Thessaloniki (800,764), Patra (185,626), Iraklio (144,642), Volos (124,591), and Larissa (124,394)