BRUSSELS — Europe’s long standoff over Greece’s debt moved into an unpredictable stage today, with tensions reaching their highest levels yet and the risk growing rapidly that Greece could crash out of the European currency.

On Saturday, eurozone finance ministers meeting in Brussels rejected Greece’s request to extend its existing bailout program past a Tuesday deadline. Greece wanted the extension so it could hold a national referendum July 5 to let voters decide whether the country should accept bailout aid under terms the government of Prime Minister Alexis Tsipras bitterly opposes.

Then early today, lawmakers in Athens voted 178-120 to go forward with the referendum, after a day in which many Greeks lined up at cash machines to withdraw money from banks out of concern that a fresh financial crisis could be at hand.

Addressing Parliament before the vote, Tsipras defended his decision to call a plebiscite, saying it would “honor the sovereignty of our people,” and called on Greeks to say a “big ‘no’ to the ultimatum,” referring to the creditors’ proposal for a deal. He added that his government would “respect the outcome, whatever it is.”

After five months of grinding negotiations, Tsipras’ surprise referendum gambit — announced early Saturday on national television while many ordinary citizens were asleep — left unclear whether he was seeking a final bit of leverage for a last-minute deal or was essentially calling an end to the negotiations.

Negotiators in Brussels had been racing the clock to reach a deal by the end of the day Tuesday, when the European part of the current bailout program for Greece expires.

At the conclusion of the meeting Saturday, the eurogroup, in a statement, said the end of the current program “will require measures by the Greek authorities” to “safeguard stability of the Greek financial system” in what amounted to a thinly veiled reference to the need for Athens to plan imposing capital controls to stem the flight of deposits.

Uncertainties now abound in Brussels, Athens and the other European capitals, where leaders were weighing the costs of making last-minute concessions to Greece or possibly risk Greece becoming the first country to abandon the euro currency.

“We are in a pretty big mess right now,” said Guntram Wolff, director of Bruegel, a research institute in Brussels.