By Pavel Polityuk and James Mackenzie MARIUPOL/LVIV/KYIV, Ukraine (Reuters) – Ukraine defied Moscow’s demand for its soldiers to lay down arms before dawn on Monday in besieged Mariupol where hundreds of thousands of civilians are cowering from Russian bombardments laying waste to their city. Russia’s military had ordered residents of the southeastern port to surrender by 5 a.m., saying those who did so could leave, while those who stayed would be handed to tribunals run by Moscow-backed separatists. President Volodymyr Zelenskiy’s government responded that it would never bow to ultimatums and…