"We ask you to reject it with all the might of your soul, with the greatest margin possible," he said on Monday.
"The greater the participation and the rejection of this deal, the greater the possibility will be to restart the negotiations to set a course of logic and sustainability," he said on state television.
The leftist prime minister also made clear that Greece was unable to make the $1.8bn payment due to the International Monetary Fund on Tuesday, the same day its international bailout programme expires.
"[How] is it possible the creditors are waiting for the IMF payment while our banks are being suffocated?" he said in a late-evening interview on ERT television.
At least 20,000 defiant supporters of Tsipras' left-wing Syriza party packed the main avenue in front of parliament on Monday night, rallying behind his call to reject the package and more austerity.
Tsipras called for Sunday's referendum over the weekend, arguing that demands for tougher austerity measures could not be accepted after more than five years of recession.
