Antonis Samaras, leader of center-right opposition party New Democracy, responded to Tsipras’ comments, saying the prime minister "persists in lying," and that his Syriza party “always wanted to take the country out of the euro.”
Meanwhile, communist party KKE has urged its followers to ditch the ballot altogether in favor of a write-in campaign voicing opposition to both "yes" and "no" choices, as well as any proposal put forth by Tsipras’ government.
"The government is raising a false dilemma" through the referendum, said KKE parliament member Kostas Papadakis, arguing that both Syriza and European negotiators are pushing "anti-people" austerity measures.
A poll conducted Tuesday and Wednesday and published in To Ethnos newspaper Friday showed the two sides in a dead heat. It also showed an overwhelming majority — 74 percent — want the country to remain in Europe's joint currency, the euro, compared to 15 percent who want a national currency.
Of the 1,000 respondents to the nationwide survey by the ALCO polling firm, 41.5 percent will vote "yes" and 40.2 percent "no," well within the margin of error of 3.1 percentage points. Another 10.9 percent were undecided and the rest said they would abstain or leave their ballots blank.
Tens of thousands of Greeks on both sides of the debate tried to sway the undecided in rallies Friday evening, held 875 yards apart in central Athens. Police estimated that 25,000 people attended the "no" rally and 17,000 the "yes" rally. KKE held a separate rally in central Athens on Thursday.
At the “no” rally, Tsipras told throngs of supporters that the referendum came down to a decision about living “in Europe with dignity.”
Emotions flared and brief clashes broke out between a group of protesters and police at the tail end of the “no” rally just as it was getting underway.
Greek police used pepper spray to deter several dozen anti-establishment protesters from throwing rocks and smashing property. However, the rallies were largely peaceful.
Sunday’s vote could be the most important in Greece's modern history, but the question is unclear and many voters are confused about what's at stake.
On Friday, The Council of State, the country's highest administrative court, rejected a motion brought by two private citizens asking the court to bar the referendum.
Sunday's vote "is invalid because it expressly violates the constitution, which stipulates that a referendum cannot take place on economic matters," Spyridon Nicolaou, one of the two filing the motion, told The Associated Press, before the ruling.
"But it's also invalid because it doesn't incorporate the text of the documents on which the Greek people are called on to decide. Would anyone from Evros (in far northeastern Greece) know the specific documents?"
A separate group had filed a counter-motion arguing that the Council had no jurisdiction over the vote and that such a referendum could be held on "crucial national matters."
Council President Nikos Sakellariou said Friday "the referendum will proceed normally."
Much of the ambiguity arose from the complicated question on the ballot paper:
"Must the agreement plan be accepted which was submitted by the European Commission, the European Central Bank and the IMF to the Eurogroup of 25 June, 2015, and is comprised of two parts which make up their joint proposal? The first document is titled `reforms for the completion of the current program and beyond' and the second `Preliminary debt sustainability analysis.'"
Voters are asked to check one of two boxes: "not approved/no" and — below it — "approved/yes."
Apostolos Foutsitzis, a 43-year-old medical scanner operator in the northern city of Thessaloniki, said he was confused by the question, but plans to vote "yes" because he wants Greece to remain in Europe.
"The referendum is unclear in the way it is being phrased, so I interpret this ambiguity as meaning we might stay in Europe or not," he said.
Al Jazeera and wire services. Lauren Zanolli also contributed to this report.