YANGON, Burma—Sightings of armored personnel carriers in the biggest city of Burma (also known as Myanmar) and an internet shutdown raised political tensions late Sunday, after vast numbers of people around the country flouted orders against demonstrations to protest the military’s seizure of power. Public concern has already been heightened for the past few nights by what many charge is the military’s manipulation of criminals released from prison to carry out nighttime violence and stir up panic. Ambassadors from the United States and Canada and 12 European nations called on Burma’s security forces to refrain from violence against those “protesting the overthrow of their legitimate government.” They condemned the arrests of political leaders and activists as well as the military’s interference with communications. “We support the people of Myanmar in their quest for democracy, freedom, peace, and prosperity,” they said in a joint statement issued late Sunday night. “The world is …