Greek PM Alexis Tsipras has called on voters to reject "blackmail" when they vote in a bailout referendum on Sunday.
In a TV address, he insisted Greece's presence in the EU was not at stake.
There were short-lived scuffles between police and protesters as "Yes" and "No" rallies began in Athens on Friday.
A Greek court has rejected a challenge to the legality of the referendum, in which voters will decide whether they support the terms of further international loans.
It comes after tough talks with creditors, and EU leaders have warned that a "No" vote could see Greece leave the eurozone.
Greece's economy is already being squeezed after its bailout programme ran out on Tuesday.
Banks have been shut and limits imposed on cash withdrawals.
The head of Greece's banking association has warned that although the banks have enough funds until Monday, they will be dependent on the European Central Bank thereafter.
Claims by Greek politicians that a "No" vote will strengthen their hand in bailout negotiations have been rebuffed by European leaders.
Both EU Commission President Jean-Claude Juncker and Jeroen Dijsselbloem - head of the Eurogroup of finance ministers - have insisted a "No" vote will weaken the Greeks' position and that even a "Yes" vote will not mean a deal is easy to agree.
Several European officials have complained in strong terms about Greece's abrupt decision to hold a referendum on the terms of a bailout offer that they say is no longer on the table.
But in a ruling on Friday, Greece's top administrative court rejected an appeal lodged by two individuals who had argued that it was illegal to hold popular votes on fiscal matters.
Thousands of "Yes" and "No" supporters gathered in separate squares of central Athens on Friday, and police threw stun grenades in brief scuffles with "No" protesters.
In his TV address, Mr Tsipras referred to an IMF report published on Thursday which said Greece would need an extra €50 billion over the next three years to stabilise its finances and repeated suggestions that Greece needed debt relief.
The recent negotiations were over the release of a final tranche of bailout funds of €7.2 billion.
Greece's mountain of €323 billion debt was "not sustainable", Mr Tsipras said. He said he had called for a 30 percent "haircut" off the debt and a 20-year grace period for the rest.
Mr Tsipras urged voters to reject the "sirens of scaremongering".
But he added: "Whatever we choose... Come Monday we are all together."
Mr Tsipras faces vocal opposition at home - with opposition leader Antonis Samaras calling on "every Greek man and every Greek woman above and beyond parties" to vote "Yes", along with several deputies from Syriza's own coalition partner, the Independent Greeks.