Three opinion polls carried out during Greece's bailout referendum, which could affect the country's future in the eurozone, indicate the "no" vote will win.
Prime Minister Alexis Tsipras called Sunday's referendum last weekend, urging voters to reject creditor reform proposals.
Opposition parties and many European officials have warned that a "no" vote, however, could endanger Greece's position in Europe's joint currency, the euro.
The vote was held amid banking restrictions imposed last Monday, with Greeks lining up at ATMs across the country to withdraw a maximum 60 euros ($66) per day. Banks have been shut all week, and it is uncertain when they will reopen.