Greek Prime Minister Alexis Tsipras is urging voters to vote "no" in Sunday's referendum on whether the country should accept European bailout terms.
Tsipras said Friday a "no" vote would give Greece a chance to "live in dignity in Europe" and give Athens more bargaining power in negotiations with European creditors.
He addressed a rally of 25,000 people supporting a "no" vote in central Athens, with one poster reading, "On July 5, We Are Writing History / No Vote."
As the rally got underway, police threw stun grenades and clashed with protesters before the situation quickly calmed down. Greek police say two people were detained.
A rival rally of 20,000 people who want a "yes" vote was held less than a kilometer away outside Panathenaic Stadium. Protesters there shouted pro-European slogans and voiced fears of a so-called "Grexit" from the eurozone if the referendum does not pass.
European leaders have warned that a “no” vote in the referendum would amount to Greece becoming the first country to leave the 19-nation eurozone during the common currency's 16-year history.
Speaking to reporters Friday in Luxembourg, European Commission President Jean-Claude Juncker said that a ‘no’ vote in Sunday’s referendum means that Greece’s position will be “dramatically weakened.”
"The program has come to an end, there are no negotiations under way, if the Greeks will vote ‘no’, they have done everything but strengthening the Greek negotiation position. The Greek negotiation position will be dramatically weakened by a 'no' vote," Juncker said.
Posters reading "We vote en masse, no the agreement'' are seen under Bank of Greece logo in Athens, July 3, 2015.
Luxembourg has just assumed the rotating presidency of the European Union for the next six months.
Supporters of the "yes" vote -- meaning Greece accepts creditors' proposals for more austerity measures in exchange for more loans -- say the referendum amounts to a vote on whether Greece wants to stay in the Eurozone.
Meanwhile, Greece's highest administrative court, the Council of State, is expected to decide on Friday on a motion filed by two private citizens asking the court to rule Sunday's referendum illegal.
A separate group filed a counter-motion supporting the referendum's legality.