A legal challenge against Greece's critical bailout referendum has been rejected, clearing the way for the vote to go ahead on Sunday.

The Council of State, the country's highest administrative court, threw out a mation brought by two private citizens asking the court to rule the referendum illegal.

Spyridon Nicolaou, one of the two filing the motion, had claimed the would be invalid "because it expressly violates the constitution, which stipulates that a referendum cannot take place on economic matters".

"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 north-eastern Greece) know the specific documents?" he added.

A separate group filed a counter-motion supporting the referendum's legality. One backer, Dimitris Belantis, said the Council of State has no jurisdiction over the matter and the constitution stipulates popular votes can be held on "crucial national matters".

Simultaneous rallies are being held in Athens supporting Yes and No answers to a murky question in what an opinion poll suggests could be a very close vote.

Prime minister Alexis Tsipras called the referendum last weekend, asking Greeks to decide whether to accept creditors' proposals for more austerity in exchange for more loans - even though those proposals are no longer on the table.

He said a No vote would put him in a stronger position to seek a better deal for Greece within the 19-nation eurozone to reduce its €320 billion national debt and make payments more sustainable.

In a televised address to the nation, Mr Tsipras urged Greeks to vote "no to ultimatums, divisions and fear". He emphasised that Sunday's referendum is not a vote on whether Greece will remain in the euro.

But opposition parties, and many European officials, say a No vote would drive Greece out of the euro and into an even more impoverished future.

Finance minister Yanis Varoufakis said an agreement with the country's creditors "is more or less done" and that the only issue left is debt relief.

But the head of the eurozone finance ministers' group, Jeroen Dijsselbloem, rejected those comments, saying negotiations are not still going on.

"There are no new proposals from our side and, whatever happens, the future for Greece will be extremely tough," Mr Dijsselbloem said. "To get Greece back on track and the economy out of the slump, tough decisions will have to be taken and every politician that says that won't be the case following a No vote is deceiving his population."

A poll conducted on Tuesday and Wednesday and published in To Ethnos newspaper today showed the two sides in a dead heat. It also showed an overwhelming majority - 74 per cent - want the country to remain in the euro, compared with 15 per cent who want a national currency.

Of the 1,000 respondents to the nationwide survey by the ALCO polling firm, 41.5 per cent will vote Yes and 40.2 per cent No, well within the margin of error of 3.1 points. Another 10.9 per cent were undecided and the rest said they would abstain or leave their ballots blank.

Both sides were trying to sway the undecided in rallies this evening, to be held 900 yards apart in central Athens. Mr Tsipras is to speak at the No rally in the capital's main Syntagma Square outside Parliament, while Yes supporters will gather at the nearby Panathenian Stadium, where the first modern Olympics were held in 1896.

The vote could be the most important in Greece's modern history, but the question is unclear and many voters are confused about what is at stake.

Much of the ambiguity arises from the complicated question on the ballot paper.

It says: "Must the agreement plan be accepted which was submitted by the European Commission, the European Central Bank and the International Monetary Fund 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 programme and beyond' and the second 'Preliminary debt sustainability analysis'."

Voters are asked to check one of two boxes: "not approved/no" or "approved/yes".