Greek citizens queued at polling stations on Sunday to cast their ballots in a historic referendum to choose whether or not to accept a debt deal proposal tabled in late June by the country's lenders.The outcome may have a decisive impact on the country's future course and determine whether Greece will avert a looming disorderly default and possible Grexit which could shake the euro zone.

Polling stations opened at 7:00 am local time and close at 7:00 pm. The first estimates on the results by pollsters are expected as the ballots close and the first official estimates by the Interior Ministry at around 9:00 pm.

Opinion surveys in recent days showed that the electorate was evenly divided between YES and NO and polling firms this time since the result was too close to call did not conduct traditional exit polls outside polling booths, but were limited to interviews over phone on Saturday and Sunday.

Casting their ballots on Sunday all political leaders urged Greeks irrespective of the results to face the next day united to overcome the crisis.

Greek Prime Minister Alexis Tsipras, who called on voters to say NO, talked about a celebration of democracy which "will defeat fear" and about an "act of determination for a better future in Greece and Europe."

President of the Greek Republic Prokopis Pavlopoulos stressed that as of Monday "regardless of the result, Greeks should move forward united on the difficult path ahead".

Main opposition conservative party leader Antonis Samaras called on Greeks to "vote YES to Greece, YES to Europe."

Greek Finance Minister Yanis Varoufakis, who has said that he will resign if YES prevails, said that Sunday's Greek referendum was an opportunity to show the entire world that "democracy and the euro can co-exist."

In a sign of the divisions between Greek people these days, Varoufakis was cheered by some voters outside the polling station, while others strongly criticized him in front of the cameras for the closed banks, capital controls and the parallel queues outside ATMs.

Mary Evangelatou, 30, was among the about 9.8 million citizens who were eligible to vote across approximately 19,000 polling stations nationwide.

"I believe that our decision is very tough, as it is not clear from the Greek government what will happen the next day. I want to believe that YES will prevail and negotiations with our creditors will continue to find a solution," she told Xinhua in a central Athens district.

"Unfortunately, the bank holiday and capital controls have created fear and panic in the society. This climate does not allow me hoping much for the future," she added.

On the other hand Alexandra Miliaressi, 35 years old, voted for NO.

"Greeks are exhausted from the continuous austerity measures from the previous governments of the past five years. That's why I believe that NO will win. With our vote we will send our message to the lenders," she told Xinhua outside the polling booth in a northern Athens district.

Pensioner Voula Karagiorgou appeared more anxious for the tensions within families and friends rather than the outcome.

"I have participated in the previous referendum in 1974 for the change of the regime from monarchy to presidential republic. I do not remember such loud shouting in my family. A father votes for YES, while a son and the neighbor for NO. I hope tomorrow whether we will be afloat or bankrupt, with the euro or drachma, we will be all together," she told Xinhua in another district of the Greek capital.

Voters are given two ballots on Sunday, one with the question of the referendum and a blank one. Citizens must write a cross either in the box under their preferred response, or on the left or right of their response.

For the referendum to be valid, at least 40 percent of the electoral body must participate.

According to unofficial estimates from the Interior Ministry shortly before the polls closed, the turnout will most likely reach 60 percent, as in the January 25 general elections this year.

Opposition parties and citizens have complained that the question is misleading and some people do not quite understand what they are actually voting for or against.

Tsipras who called the referendum a week ago as five month negotiations with lenders had hit an impasse, said that Greeks are asked to choose whether to accept a harsh and non viable new austerity and reforms package as proposed by creditors or push for a deal on better terms in exchange of further funding to help keep the country afloat in the euro zone.

His Leftist government urged people to choose NO to strengthen its bargaining position as of Monday.

Tsipras reassured that on July 7 Greece will sign a deal and the banks which closed last Monday, when capital controls were imposed after the European Central Bank cut off emergency liquidity aid following his surprise call for the referendum, will reopen.

His critics argued that the real question, as interpreted by creditors, was whether Greeks want to remain in the euro or not, and warned of a high stakes gamble with catastrophic consequences for the economy and society if NO prevails.

Since July 1 Greece is already in arrears to the International Monetary Fund and without the safety net of the bailout program that kept it afloat over the past five years, is teetering on the brink of bankruptcy.

Officials and analysts in Greece and abroad warned that even if YES wins, banks may not reopen on Tuesday, ATMs will run out of cash and Greek citizens will face more hardships before any deal is reached and liquidity is restored with lenders' help.

The Greek referendum which according to the Interior Ministry cost about 20 million euros, attracted 699 media representatives from 40 countries from all over the world to Athens.