Protesters marched in Myanmar on Monday in defiance of a crackdown by security forces that killed at least 18 people a day earlier, as calls grew for a more united international response after the worst violence since a coup one month ago.

Clashes took place in various parts of the country on Sunday and police opened fire on crowds in the biggest city of Yangon, after tear gas and warning shots failed to clear protesters demanding the restoration of Aung San Suu Kyi's government...