YANGON, Myanmar—Police in Burma (also called Myanmar) repeatedly used tear gas and rubber bullets Tuesday against crowds protesting last month’s coup, but the demonstrators regrouped after each volley and tried to defend themselves with barricades as standoffs between protesters and security forces intensified. Burma authorities have escalated their crackdown on the protests in recent days, making mass arrests and firing into the crowds. The United Nations said it believed at least 18 people were killed on Sunday by security forces. Foreign ministers from Southeast Asian countries were meeting Tuesday to discuss the increasingly volatile crisis. Despite the crackdown, demonstrators have continued to flood the streets—and are beginning to more rigorously resist attempts to disperse them. Hundreds, many wearing construction helmets and carrying makeshift shields, gathered in Burma’s largest city of Yangon, where a day earlier police had fired repeated rounds of tear gas. They dragged bamboo poles and debris to form barricades, …