MYANMAR security forces fired shots and arrested about 30 people at an anti-coup rally in the country's second-biggest city yesterday, witnesses said, as protesters defy a months-long crackdown by a junta struggling to impose order. Chaos erupted in Mandalay, a hotbed of anti-military sentiment, when plainclothes police emerged from vehicles minutes into a protest, firing guns and beating demonstrators who fell as hundreds fled, according to four witnesses, who said they saw about 30 people arrested. "There are no words to describe their cruelty," Aung Pyae Sone Phyo, 21, a protest leader, told Reuters. "They used enormous force to crack…