Greeks are hoarding cash and food amid mounting fears the economy could collapse, cracking open their wallets only to stock up on essentials and stripping supermarket shelves in the process.

Mothers, elderly men and university students were spotted on Saturday pushing heavily overloaded trolleys or coming out of shops weighed down by bags of food, with essentials such as sugar, flour and pasta top of the list.

In the well-off area of Glyfada in Athens residents appeared to have panicked, thrusting everything from vast rolls of toilet paper to multiple packs of lentils into their carts.

"Most people are buying food now because they fear the worst," said Andreas Koutras, a 51-year old who works in finance, referring to a referendum on Sunday on Greece's bailout which could seal its financial fate.

Prime Minister Alexis Tsipras has urged people to vote 'No', insisting that rejecting a bailout deal offered by the austerity-hit country's international creditors will put it in a stronger negotiating position.

His right-hand man, Finance Minister Yanis Varoufakis, has promised banks will re-open after the vote - but with so much uncertainty surrounding Greece's future, many doubt him.

Nikos Archondis from the Panhellenic Exporters Association (PEA) told AFP "certain supermarkets are very concerned because they cannot forecast how the situation will evolve".

Stocks of meat, cheeses, fruits and vegetables "risk running low in the following weeks", he said.

Reports that medicines were also flying off the shelves were supported by pharmacist Yannis Triantaphilou.

Priorities were "food and medicines" and he had seen "an increase of customers in the pharmacy".

Although the boost in business was welcome, Triantaphilou said if the banks did not re-open on Tuesday, "I don't know how we are going to work, if companies will provide medicines".

With anti-bailout 'No' voters tied neck and neck with the 'Yes' camp, fears a financial disaster may be brewing were compounded by frustration over the decision taken by many shops to refuse card payments.

With government-imposed capital controls capping ATM withdrawals at 60 euros ($A87) per day, the number of banknotes in circulation has dropped dramatically, especially the smaller denominations.

Customers want to save their cash, but businesses are also desperate to get their hands on it.

Like many Greeks, taxi driver Theodor Veletzas said he believed there was more at stake than a new deal, and the situation risked getting worse.

The referendum, he insisted, would effectively decide whether his country stayed in the eurozone or not. And he feared what would happen if the 'No' camp won.

"There are no banknotes in circulation. Shops are closed and people have no notes with which to buy food. I hope people will vote on Sunday to stay in Europe. We don't want another adventure", he said.