Huge rival rallies are expected in central Athens late Friday to galvanise support for each side in Sunday's plebiscite.
Many Greeks, who are struggling under capital controls that have limited daily ATM withdrawals to just 60 euros ($67), fear EU leaders' warnings that a 'No' vote could mean Greece's exit from the euro -- a Grexit.
The sense of building crisis was heightened further by a eurozone emergency fund officially declaring Greece to be in default.
Tsipras insisted that his decision to stop debt negotiations last week and call the referendum "does not mean a break with Europe".
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He called on creditors to restructure Greek's unsustainable 323-billion-euro debt mountain by forgiving 30 per cent of the debt owed to them and allowing a 20-year grace period for repaying the rest.
Confusion is widespread over the very technical question posed in the referendum, compounding concerns over what the post-vote consequences might be.
An Alco institute poll found 44.8 per cent of Greeks intend to vote 'Yes' and 43.4 percent are for 'No'. A Bloomberg survey for Greece's Macedonia University was equally split, showing 43 percent to vote 'No' and 42.5 percent 'Yes'.
European Commission chief Jean-Claude Juncker warned that Greece's negotiating position, far from being strengthened, would be "dramatically weakened" in the event of a 'No'.
Even if the 'Yes' vote wins, there would still be "difficult" negotiations ahead, he added.
Greek voters are confronted with a referendum question that has stumped many.
Eurozone officials have firmly said that the "deal" referred to expired on Tuesday -- the same day Greece failed to make a 1.5-billion-euro payment to the IMF, becoming the first developed country ever to do so.
Some voters who initially backed the government have swapped sides ahead of Sunday's ballot.