CARACAS, Venezuela—A gun-toting retired Venezuelan general has become a folk hero to the country's opposition and galvanized a protest movement by defying the government of President Nicolás Maduro, engaging in a Rambo-like standoff with security forces sent by the president to arrest him. Angel Vivas has been holed up in his home in a hilly Caracas suburb since last weekend, when Mr. Maduro, on live television, ordered his arrest for having backed student protests that have convulsed this oil-rich nation. When black-clad officers from military intelligence went to the retired Army general's house Sunday morning, he emerged wearing a flak...