By Stelios Bouras And Alkman Granitsas 

ATHENS-- Alexis Tsipras, sworn into office as Greece's new prime minister a day after his radical leftist Syriza party won a resounding election victory, swiftly forged a coalition government with the aim of shedding European-imposed austerity policies.

Syriza has little in common with its coalition partner--the small, right-wing Independent Greeks party--other than a fierce opposition to the austerity measures Greece embarked on in exchange for bailouts from its eurozone partners and the International Monetary Fund. Still, the common bond on that front signals tough negotiations with Greece's creditors over its debt repayments in the months ahead.

Together, Syriza and Independent Greeks will jointly control 162 seats in Greece's 300-seat legislature. The Independent Greeks are also expected to hold at least one cabinet position in the new government, the details of which are likely to be unveiled on Tuesday.

Mr. Tsipras, who was sworn in as Greece's new prime minister in the afternoon, promised that he would give his all "to protect the interests of the Greek people."

Syriza staked its election campaign on repudiating the steep budget cuts and tax increases that Greece agreed to in exchange for a financial rescue. It has promised, first, to deliver a spending package aimed at Greece's struggling poor, and then to use money now earmarked for debt payments on social programs in Greece.

But the new government faces a race against the clock: In July and August Greece must repay some EUR7 billion ($7.9 billion) in bonds held by the European Central Bank. Without fresh aid from its creditors, Greece doesn't have the cash to pay off those bonds.

While Syriza and Independent Greeks have different policies on issues such as illegal immigration and public order that could test the unity of the coalition, their joint opposition to Europe on austerity could carry them through the first delicate months of negotiations.

"There are significant differences between them, but they could take some time to appear. This may provide them with time to secure benefits from creditors which would then be pumped directly into the economy," said John Dimas, at STR, an Athens-based communications consultancy.

Mr. Tsipras's coalition ally, the Independent Greeks' party leader Panos Kammenos, is in many ways his polar opposite: a nationalistic, right-wing, archconservative, with strong anti-immigrant views and a deep antipathy to Germany.

Known for his conservative social views, the 49-year-old Mr. Kammenos doesn't accept the separation of church and state in Greece and opposes civil unions for homosexual couples. Mr. Kammenos opposes giving Greek citizenship to non-Greeks--including children born on Greek soil--and wants to see taxes on the rich lowered as a way to stimulate investment. Those are all views that are at odds with coalition partner Syriza.

On foreign affairs, Mr. Kammenos is known for his hard-line stance. He is a harsh critic of rival Turkey(and has been declared persona non grata by the Turkish government)and has criticized Syriza for kowtowing to Ankara on issues relating to the Muslim minority in Greece.

But Mr. Kammenos is, above all, a vocal opponent of the austerity program Greece has implemented over the past five years in exchange for two international bailouts. He founded Independent Greeks in early 2012 after breaking with his former party--the center-right New Democracy--after the party switched positions and backed the austerity program. In interviews he has referred to German economic hegemony over Europe as "the Fourth Reich."

And like many Greeks, Mr. Kammenos blames Berlin for prescribing the bitter economic medicine Greece has been forced to swallow that has left a quarter of Greeks jobless, and bankrupted tens of thousands of small businesses.

Pointedly, Mr. Kammenos launched Independent Greeks in the central Greek town of Distomo: a place steeped in historical symbolism for many Greeks. In 1944, the town was the site of a World War II Nazi atrocity.

The pressure to compromise with Greece's creditors will be intense for the new coalition. Under the bailout program's rigorous schedule, Greece is required to complete a review of its progress with the so-called troika of bailout inspectors by the end of February. Mr. Tsipras has said he doesn't recognize the troika's authority.

European officials said that it was too early for ministers to discuss possible points of compromise with a Syriza government, even though the election outcome was much clearer than many had expected.

Jeroen Dijsselbloem, the head of the group of eurozone finance ministers, said that the minsters' group had been slated to discuss a possible extension of Greece's current bailout program. But he warned that there was little appetite to forgive the country's official debt.