Donald Tusk's government sworn in by president Duda

Following years of disputes between Warsaw and Brussels under the previous government, led by the Law and Justice (PiS) party, Tusk's appointment has raised hopes of smoother relations with the rest of the European Union.

In a speech to parliament on Tuesday, centrist Tusk, who as European Council president from 2014 to 2019 chaired summits of EU leaders, vowed to secure billions of euros for Poland that have been frozen by the EU over concerns about the rule of law.

His task could be complicated by judges appointed under PiS reforms that critics said undermined the independence of the courts and by the power to veto laws held by President Andrzej Duda, a PiS ally.

Duda has previously spoken in highly critical terms about Tusk, but on Wednesday he struck a conciliatory tone following the swearing-in ceremony.

"Please be aware that I am open to cooperation," he said. "We come from different political camps, but I have found out that on important issues, such as security, we can come to an understanding."

Tusk, 66, said his government would focus on restoring the rule of law and respect for the constitution, and his government holds Duda partly responsible for reforms that it says undermined judicial independence.

"Faithfulness to the provisions of the constitution will be the trademark of our government," Tusk said, adding that he thought voters' desire to see the rule of law re-established was behind record turnout in an Oct. 15 election.

PiS came first in the election but lacked the necessary majority to form a government, paving the way for Tusk to form a government which won parliament's approval on Tuesday.

In an example of the challenges Tusk will face, Poland's Constitutional Tribunal ruled on Monday that judicial reform legislation which Poland needed to pass to access EU funds was unconstitutional.

It reached the same conclusion about penalties imposed by the EU's top court before it reaches a final ruling, known as interim measures.

Last-minute spending by the previous government has also added to strains on the budget, complicating Tusk's efforts to deliver on campaign pledges.

Tusk is expected to travel to Brussels for an EU summit that takes place on Thursday and Friday.

In addition to trying to unblock funds for Poland, Tusk will add his support to Ukraine's bid to eventually join the bloc.

With concerns growing in Kyiv about its Western allies' commitment to funding its defence against Russia's invasion, Tusk said on Tuesday that Poland would advocate for continued support.

Ukraine faces the possibility that Hungary will not give the green light for it to start EU accession talks at a Brussels summit.