More than a third of automated teller machines across Greece ran out of cash on Saturday before they were replenished as Greeks pulled out money on fears their country was set to crash out of the euro, three banking sources said.

Anxious Greeks lined up outside ATMs after Prime Minister Alexis Tsipras made a surprise call for a referendum on austerity terms demanded by lenders, throwing talks with lenders in disarray and putting Greece on the verge of a default.

About 35 percent of the ATM network - some 2,000 out of the 5,500 ATMs across Greece - ran out of euro banknotes at one point during the day and were being replenished, the bankers said. Banks were working in coordination with the central bank to keep the network fed with cash, they said.

Replenishing ATMs usually takes one to two hours per ATM, leading to the long lines, one banking source said.

Around 600 million euros was withdrawn from the banking system on Saturday, one senior banker at one of Greece's four big lenders told Reuters. A second banker estimated the outflow at more than 500 million euros.

Though that was below the level of over 1 billion euros seen on some days over the past two weeks, the figure was almost exclusively from ATM withdrawals, where the average daily limit of cash that can be taken out is 600 to 700 euros, bankers said.

"Demand for cash is definitely higher than what you see on a normal Saturday," one of the bankers said.

"This does not mean that there are lines everywhere but we are trying to keep ATMs fed with banknotes."

Minutes after Tsipras's address to the nation, small lines could be seen at some ATMs in Athens. In addition to lines at various ATMs across the country on Saturday, a line of about 40 people could be seen at an ATM inside the Greek parliament.

Greece's government has insisted that banks will reopen as normal on Monday and denied the country will have to impose capital controls to prevent the banks collapsing.

The banks depend on emergency liquidity from the European Central Bank to stay open, and senior government officials held talks with the ECB chief Mario Draghi on Saturday to ensure continued support to banks amid the crisis. (Writing by Deepa Babington; editing by Ralph Boulton)