ATHENS — The leaders of the two parties remaining in Greece’s shaky coalition government urged lawmakers on Saturday to approve a debt deal to avert a default as protesters took to the streets for a second day ahead of a crucial parliamentary vote.

Echoing the comments of Prime Minister Lucas Papademos in his cabinet address on Friday night, the political leaders acknowledged that the austerity measures set out by Greece’s lenders were harsh, but warned that the alternative — near certain bankruptcy — would be far worse.

Greece’s so-called troika of foreign lenders — the European Commission, European Central Bank and International Monetary Fund — has demanded sweeping austerity measures in exchange for about $170 billion in bailout money that Greece needs in order to avert default. The troika has also made passage of the measures a condition for sealing a deal in which private creditors will take voluntary losses of up to 70 percent of Greek debt.

After a wave of defections from the cabinet, Mr. Papademos told lawmakers on Friday that they must approve the austerity measures demanded by Greece’s creditors — including a 22 percent cut in the benchmark minimum wage and public-sector layoffs — or the country would suffer a disorderly default with social dislocation and would eventually leave the euro zone. After a five-hour meeting, the cabinet approved the package, sending it on to Parliament for final approval.

Antonis Samaras, the conservative leader of New Democracy who is likely to be the next prime minister of Greece, said that elections should also be called immediately after the completion of a debt-swap deal being negotiated between the government and private creditors. And in a bid to discourage would-be dissenters, Mr. Samaras has warned members that anyone refusing to back the measures would not be a candidate in the coming elections.

Meanwhile George Papandreou, the former prime minister and leader of the Socialist party, which has seen its support plummet, appealed to his deputies to put party interests aside for the good of the nation. He compared Greece to a parched traveler. “We’ve gone too far to turn back now,” he said. “We have reached the source. The country must drink water.”

While several members of Parliament from the coalition parties — the Socialists and the center-right New Democracy — have indicated that they will vote against the deal, the dissent is far from the level needed to prevent it from passing. Still at least six lawmakers from New Democracy have said they will vote no and the number of dissenters among the Socialists is believed to be close to 20 though only a handful have expressed their intentions. Three members of Parliament have resigned from the Socialist party.

Outside Parliament, union leaders held a demonstration that attracted about 3,500 protesters, according to the police, who said there had been no arrests and could not confirm media reports of two minor injuries to demonstrators. Local media reported that at least 4,000 officers were on duty. Another protest was expected on Sunday.

Ahead of Sunday’s vote, Mr. Papademos is scheduled to address the nation on Saturday night, appealing to Greek citizens to accept the new measures.

The prime minister’s comments Friday kicked off what was expected to be a long and chaotic weekend of brinkmanship, with Greek politicians fighting for their survival in the face of unpopular austerity measures and European leaders demanding more concessions in a climate of growing urgency — and mistrust — between Greece and its foreign lenders.

On Saturday, the Greek finance minister Evangelos Venizelos said the troika of foreign lenders had proposed allotting an additional $20 billion to prop up Greek banks. “Their aim is to ensure there is funding through 2015 to allow a smoother adjustment,” Mr. Venizelos told a meeting of Socialist deputies.

Nearly two years after Greece’s first bailout, Athens and its lenders are at a dangerous impasse. Europe has lost confidence that the Greek government has the will or capacity to follow through on its commitments to structural changes. Greeks, whose standard of living is dropping precipitously with no end in sight, have lost confidence that the bailout will actually save the country from default.