YANGON, Burma—Police cracked down on demonstrators opposing Burma (also known as Myanmar)’s military coup, firing warning shots and shooting water cannons to disperse crowds that took to the streets again Tuesday in defiance of new protest bans. Reports of many injured demonstrators drew strong concern from the U.N.’s office in Burma. “According to reports from Nay Pyi Taw, Mandalay and other cities, numerous demonstrators have been injured, some of them seriously, by security forces in connection with the current protests across the country,” the U.N. said. “The use of disproportionate force against demonstrators is unacceptable,” said Ola Almgren, the U.N. resident coordinator in Burma. Water cannons were used in Mandalay, Burma’s second-biggest city, where witnesses said at least two warning shots were fired in early attempts to break up the crowd. Gunfire could be heard on videos from the city, some of which showed riot police flailing wildly with their …