ATHENS — Greece ushered in a new government on Wednesday that will put it back at Europe’s bargaining table, ending a seven-week leadership vacuum that had destabilized this already fragile nation and cast a shadow over the future of the entire euro monetary union.

Antonis Samaras, the leader of the New Democracy Party, was sworn in Wednesday afternoon as prime minister in a televised ceremony conducted by Archbishop Ieronymos of the Greek Orthodox Church. Mr. Samaras is preparing to work with two other parties that garnered substantial votes in Sunday’s pivotal parliamentary elections.
But whether his coalition government will last more than a few months, or succeed in renegotiating some of the harsh austerity terms of Greece’s multibillion-euro bailouts with its European partners, remained an open question.
Together with the Greek Socialist party known as Pasok and the Democratic Left, a small party that won only 16 percent of the vote, Mr. Samaras is maneuvering to form an airtight majority coalition in Parliament that he hopes will resist opposition from the popular far-left Syriza party, which has said it will fight if the new government does not repudiate the most onerous terms of Greece’s loan deals.
In comments to reporters after being sworn in, Mr. Samaras, emphasized the need for “patriotism, national unity and trust that with the help of God we can ensure that the Greek people emerge from the crisis as soon as possible” He said he would press the members of his new cabinet — expected to be named by late Wednesday — to work hard, with the aim of giving Greeks “tangible hope.”
During the handover with the outgoing caretaker prime minister, Panagiotis Pikrammenos, Mr. Samaras said, “I am fully aware of the critical moments we face as a country,” adding that the Greek people were “injured” and needed “healing.” Mr. Pikrammenos, for his part, wished Mr. Samaras good luck and said he was assuming power “at a difficult and historic crossroads and will have many battles to fight at home and abroad.”
In a written statement issued after Mr. Samaras was sworn in, Syriza said the new government would not be able to renegotiate the country’s loan deal, noting that the administration had been “built on the basis of continuing the policies of the memorandum,” a reference to the debt deal. “The political balance within the government suggests that it is not willing or able to strive for an essential renegotiation of the terms of the debt deal nor to doubt the dominant neoliberal austerity policies within a Europe that is trying to change its architecture,” the statement added.
With the economy in a downward spiral and the government bleeding cash, Mr. Samaras’s government can “incrementally develop a certain sense of stability and certainty, but it will always be vulnerable to any shock — whether political or economic,” said Marco Vicenzino, director of the Global Strategy Project, a geopolitical risk analysis firm based in London.
Evangelos Venizelos, the Pasok leader and a former finance minister who negotiated Greece’s second debt deal with foreign creditors, said George Zannias, the outgoing finance minister, would represent Greece at a European finance ministers’ summit meeting on Thursday. It is expected that the president of the National Bank of Greece, Vassilis Rapanos, will assume the finance portfolio in the new cabinet.
Mr. Venizelos said he and the leaders of the other two parties in the coalition would meet with Mr. Zannias Wednesday night for a briefing ahead of the finance ministers’ meeting.
“We have to prepare very well for the European summit; it will be the first battle to change the program and return to growth,” said Mr.
Venizelos, referring to Greece’s debt deal with creditors. He again criticized Syriza, which came second in Sunday’s elections, for not joining a “national negotiating team.”
Earlier on Wednesday, Mr. Samaras met with Mr. Kouvelis. In a televised statement after the talks, Mr. Kouvelis confirmed his party’s support for a New Democracy-led coalition. He said he expected an agreement on a common policy platform within few hours and the composition of the new government to be announced by Thursday at the latest.
Earlier in the day, Mr. Venizelos met with his party’s legislators to discuss his proposals for the form that the Socialists’ participation in a coalition should take. On Tuesday, he said former top-ranking Socialist ministers and current legislators should not join the cabinet to avoid being associated with any additional austerity measures, comments that drew criticism from some party officials. Leaders of the Democratic Left also hinted that they might seek to avoid responsibility for decisions of the new government by limiting their participation in the new cabinet.
Mr. Venizelos noted that many of the tough terms of the bailout that he helped work out while serving as finance minister had been “imposed” on Greece during the first phase of the negotiations, when the critical goal was to seal a deal quickly to ensure that Athens did not run out of money to pay its bills. He said the agreement was always seen as open to revision.
The new government will face a daunting double mandate to enforce Greece’s loan agreement with its foreign creditors — the European Central Bank, the International Monetary Fund and the nations of the European Union — while renegotiating enough of the bailout terms to keep the government in power in the face of growing social unrest among the rapidly unraveling middle class. It must also reassure investors enough to slow the flight of deposits from Greek banks.
“Greek banks may be small,” said Carl Weinberg, the chief economist at High Frequency Economics, a consulting firm in Valhalla, N.Y., “but if the public doesn’t see that government authorities can ensure that the banks are safe at all times, then the banking system won’t be stable.”
Greece’s official creditors had made it no secret that they wanted Mr. Samaras to prevail over Syriza, with its calls to tear up the loan agreement.
The creditors have indicated that they are willing to talk about changes to the agreement, but it is unclear how far Europe’s leaders, particularly Chancellor Angela Merkel of Germany, are prepared to go in making concessions.
Rachel Donadio contributed reporting.