By Maria Tsvetkova KYIV (Reuters) -The mayor of Kyiv imposed a curfew on Thursday after Russia invaded Ukraine and the capital echoed throughout the day to the sound of gunfire, sirens and explosions. Many people tried to flee for safer areas, snarling traffic. Black smoke rose from a defence ministry headquarters, a missile caused minor damage after landing in the city and officials said a nearby airport was attacked. But people were defiant as Mayor Vitaly Klitschko declared a curfew from 10 p.m. until 7 a.m., saying transport would be halted overnight and subway stations used as round-the-c…