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 EU membership was not at stake. He later told cheering supporters to vote "No" to "live with dignity in Europe".
There were scuffles with police as both sides held rallies in Athens.
A Greek court has rejected a challenge to the legality of the referendum, in which people will vote on 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.
It is a scene that speaks of suffering and exhaustion, and one that may well become the lasting image of the Greek debt crisis.
An elderly man, sitting on the ground outside a bank, was photographed crying on Friday morning.
The image, taken in Thessaloniki by AFP's Sakis Mitrolidis, was widely shared on social media as soon as it was published.
The man, whose name was not disclosed, was helped away soon after it was taken.
His story is not known, but those of plenty of other pensioners are.
Monthly pensions have gone down to an average of €833 ($924; £594) from an average of €1,350 in 2009, according to INE-GSEE, the institute behind Greece's biggest union.
Read more: Cost of crisis to pensioners
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.
Debt 'unsustainable'
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.
Mr Tsipras urged Greeks, at a "No" rally later on Friday evening, to say a "proud 'No' to the ultimatums".
In his TV address, Mr Tsipras referred to an IMF report published on Thursdaywhich said Greece would need an extra €50bn euros ($55bn; £36bn) over the next three years to stabilise its finances and repeated suggestions that Greece needed debt relief.
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