By Lefteris Papadimas and Renee Maltezou ATHENS/BRUSSELS (Reuters) - Greek Prime Minister Alexis Tsipras called a referendum on austerity demands from foreign creditors on Saturday, rejecting an "ultimatum" from lenders and putting a deal that could determine Greece's future in Europe to a popular vote. This responsibility obliges us to respond to the ultimatum through the sovereign will of the Greek people," Tsipras said in a televised address to the nation just after midnight. Euro zone finance ministers were still due to meet at 1200 GMT in Brussels to discuss the aid offer from the euro zone and the International Monetary Fund, just after the Greek parliament meets to approve the referendum decision.