"Regardless of the outcome of the referendum, we must embark in an indissoluble unity on a difficult journey tomorrow. That is what our ancestors did in the crucial moments. That is our duty today," Pavlopoulos said after voting.
Greece owes some $270 billion of its aggregate $350-billion debt to its major international lenders — the European Central Bank, the International Monetary Fund (IMF) and some Eurozone nations.
Greece failed to transfer its $1.7-billion loan repayment to the IMF before the expiry of the previous aid package on June 30.
The decision to hold a nation-wide referendum was preceded by several rounds of talks between Athens and its international creditors on Greece's debt settlement, which resulted in no agreement.