His typically charismatic turn on the stage came as the latest polls suggested Sunday's plebiscite on Greece's latest bailout offer was too close to call, with the EU nation of 11 million people evenly divided.
Countering EU leaders' warnings that a "No" could cause Greece to crash out of the eurozone, Tsipras yesterday told the crowd of 25,000 cheering supporters: "We are not simply deciding to remain in Europe -- we are deciding to live with dignity in Europe.
Just 800 metres away, separated by police with riot shields, a rival rally of 20,000 "Yes" supporters shouted pro-European slogans and voiced fears of a so-called "Grexit" from the eurozone if Tsipras got his way.
"They cannot pretend any longer that it's not about leaving the euro," said a 43-year-old doctor who gave his first name as Nikos. "And outside the euro lies only misery."
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"It cannot go on like this. Our government is a liability," said George Koptopoulos, a 70-year-old retired university professor.
Adding to the sense of crisis, a eurozone emergency fund officially declared Greece to be in default yesterday for not making a 1.5-billion payment to the International Monetary Fund loan this week.
Greece's single biggest creditor, the European Financial Stability Facility (EFSF) said, however, it was not yet demanding immediate repayment of loans worth 145 billion euros (USD 160 billion).
The two latest voter intention polls showed growing support for a "Yes" compared to previous surveys.
Greece's top administrative court ruled late yesterday the referendum could go ahead after rejecting a challenge by two citizens who argued its question was confusing and unconstitutional.
Tsipras says the referendum is needed to force creditors to finally accept his key demand of another round of debt relief to save Greece from financial meltdown and possibly crashing out of the euro.