Myanmar police fired on protesters around the country on Sunday and at least 18 people were killed in the worst violence since the Feb. 1 military coup, the United Nations said, calling on the international community to act to stop the repression.

Crowds of demonstrators came under fire in various parts of the biggest city of Yangon after stun grenades, tear gas and shots in the air failed to break up their protests.

Across the country, protesters wearing plastic work helmets and with makeshift shields faced off against police and soldiers in battle gear, including some from units notorious for tough crackdowns on ethnic rebel groups in Myanmar's border regions.