Hours after Greece became the first advanced economy to default on the IMF, the leftist leader used a televised address to tell Greeks to vote 'No' on Sunday to creditor austerity demands.
European ministers were due later Wednesday to consider a new proposal from Athens but German Chancellor Angela Merkel has already insisted there can be no new deal before the referendum.
"A 'No' vote does not signify a rupture with Europe, but a return to the Europe of values," Tsipras told the nation, rebutting accusations from EU leaders that the plebiscite was essentially a vote on whether to stay in the union.
The radical Greek leader spoke hours after Merkel effectively ruled out all negotiations until after Sunday, saying that Europe could "calmly" await the outcome of the referendum.
More From This Section
"The world is watching us. But the future of Europe is not at stake," Merkel told Germany's Bundestag lower house of parliament.
But there were signs the crisis was opening rifts in Europe's united front, with France's leader Francois Hollande urging an "immediate agreement" after six months of stalemate with the radical-leftists in Athens.
The Council of Europe rights group meanwhile said the vote fell short of European standards.
Greece entered uncharted waters with its default on a 1.5 billion euro (USD 1.7 billion) International Monetary Fund loan, the first by an advanced economy, and expiry of its current European bailout at 2200 GMT yesterday.
It is now without external financial assistance for the first time in five years, while at home the banks will be closed all week, although around a thousand branches opened Wednesday to allow the elderly to receive pension payments.