Major rallies for both 'Yes' and 'No' votes are expected to be held in Athens ahead of Sunday's referendum on an international bailout deal.

Campaigners are racing against time to reach voters before Sunday, BBC reports. Posters for 'aye' and 'nay' have been vying for space in Athens.

The BBC noted that so far there has not been any referendum campaigning as such in Greece, with Athens only seeing a few chaotic days as supporters and opponents of the ruling Syriza party jostled for position.

Competing rallies are expected to be held in the Athens city centre today (3 July). Speakers are expected to define what the referendum is really about, BBC says.

Protestors have already taken to the streets over the past few days. Some 6,000 Greek Communist Party supporters held a demonstration on 2 July, calling on voters to cast invalid ballots while a 'No' rally took place at the city's university.

There is also uncertainty whether the referendum will even take place. Greece's top administrative court, the Council of State is due to decide on the legality of the referendum vote which is due on Sunday. The court will have to decide whether it breaches Greece's constitution, BBC said.

Further, the Human rights body, the Council of Europe has already said that the referendum would "fall short of international standards" if held as planned, given the short notice period and the lack of clarify in the question to be placed before the voters.

Greece referendum: What a \'Yes\' or \'No\' vote would mean for Europe IBTimes UK

A poll by euro2day said 47% of voters were leading towards a 'Yes' vote. A previous poll had suggested that the 'No' camp had a shrinking lead.

Greek Prime Minister Alexis Tsipras has said that a strong 'No' vote will help lead to a "better agreement" with creditors.

"Our efforts are focused on overcoming the crisis as fast as possible, with a solution that preserves the dignity and sovereignty of our people," he said.

Greek Finance Minister Yanis Varoufakis has said that he would resign if 'Yes' votes won the day. He however told BBC that he was confident a deal would be reached shortly after the referendum, allowing the banks to reopen on Tuesday.

Greek banks are scheduled to remain closed until after the referendum with capital controls covering cash withdrawals and international transfers currently in place.

A 'Yes' campaigner, former Greek Prime Minister Costas Karamanlis has said the world would consider a 'No' vote to be "a withdrawal from the heart of Europe, the first step towards euro exit."