The left-wing government is urging a "No" vote, saying Greece's European partners are bluffing when they warn that would mean a Greek departure from Europe's single currency, with unforeseeable consequences for Greece, Europe and the global economy.
Opinion polls on Friday gave the "Yes" camp, which favors accepting the bailout terms, a slender lead but all were within the margin of error and pollsters said the vote was too close to call.
Only one had the "No" vote winning, despite turnout of at least 50,000 opponents of the deal at a rally in central Athens on Friday that appeared significantly bigger than a simultaneous rally by the "Yes" camp.
"What they're asking us to accept is eternal slavery," said Ermioni Tenekidou, 54, a teacher.
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Finance Minister Wolfgang Schaeuble of Germany, Greece's biggest creditor and toughest critic, said any so-called Grexit, Greek exit from the euro zone, might only be temporary.
"Greece is a member of the euro zone. There's no doubt about that. Whether with the euro or temporarily without it: only the Greeks can answer this question. And it is clear that we will not leave the people in the lurch," he said.
But it is far from clear how a temporary exit from the 19-nation euro currency bloc might work. Some economists have raised the idea of a temporary suspension, whereby Greece would revert to a national currency for a number of years until its economy stabilized.