ATHENS, Greece —Anxious pensioners swarmed closed bank branches Monday and long lines snaked outside ATMs as Greeks endured the first day of serious controls on their daily economic lives ahead of a referendum that could determine whether the country has to ditch the euro currency and return to the drachma. Greek Prime Minister Alexis Tsipras was defiant, urging voters to reject creditors’ demands, insisting a “No” vote in next Sunday’s referendum would strengthen Athens’ negotiating hand. “We ask you to reject it with all the might of your soul, with the greatest margin possible,” he said on state television.