Greek conservative opposition leader Antonis Samaras is challenging Prime Minister Alexis Tsipras to a TV debate on the referendum.
The conservative leader told his lawmakers that he was seeking a live debate with all the parliamentary party leaders ahead of Sunday's vote.
Samaras, who lost elections to Tsipras in January, echoed warnings made by a European leaders that a "No" vote would lead to the country's exit from the euro.
He said "it would be an unprecedented catastrophe ... If you want to stay in the euro, keep banks open, back Europe, vote Yes, otherwise cast a No ballot."