Warsaw (AFP) - New nationalist President Karol Nawrocki called for a “sovereign Poland” and promised to “fight those who are pushing the nation towards decline” as he was sworn in on Wednesday.

Nawrocki, 42, a supporter of US President Donald Trump, won a June 1 election in a major blow for the pro-EU government of Prime Minister Donald Tusk, a former European Council president.

“I will be the voice of those who want a sovereign Poland that is in the EU, but a Poland that is not the EU,” he told lawmakers after taking his oath of office.

“We must fight those who are pushing the nation towards decline and degradation,” he said, citing Ignacy Paderewski, a Polish prime minister from the early 20th century.

Nawrocki, a historian and political novice, has branded Tusk’s government as the “worst” in the history of post-communist Poland.

Tusk said he was “prepared, of course” for a “confrontation” with the new president.

He also warned Nawrocki against overstepping his remit.

“If necessary, we will of course stand firm as guardians of the constitution,” he said.

Presidents in Poland can initiate as well as veto legislation, but the government is in charge of domestic and foreign policy.

In a more conciliatory message on social media, Tusk said he had already worked with three presidents in the past.

“What will it be like with the fourth? We’ll manage,” he wrote.

Relations between government and president are likely to be tense, particularly in the run-up to parliamentary elections planned for 2027.

- ‘He doesn’t grovel’ -

Outside the parliament, thousands of people came to show their support for Nawrocki.

“He doesn’t grovel before Brussels,” Jan Smolinski, 75, a retired miner, told AFP, adding: “He’s a true Pole, flesh and blood.”

Marietta Borcz, a 57-year-old dental assistant, said it was “important” to her that Nawrocki “is Catholic and will uphold Christian values”.

During the election campaign, Nawrocki ruled out easing Poland’s near total abortion ban or allowing same-sex civil partnerships.

Nawrocki greeted supporters in different parts of Warsaw and attended a mass in the capital’s Old Town.

Also outside the parliament, a small group of around 20 protesters held up black roses, saying Nawrocki’s inauguration was “a black day for Poland”.

Stanislawa Sklodowska, 72, a retired economist said his election “reflects poorly on us Poles”.

Nawrocki, who travelled to Washington to seek Trump’s backing during the campaign, won a narrow victory against liberal candidate Rafal Trzaskowski.

The result showed the high degree of political polarisation in the EU and NATO member state, a key supporter of neighbouring Ukraine.

Nawrocki has promised to be “an active president” from the start and has said he wants to “stimulate” the government with various bills.

But the government holds a parliamentary majority and analysts say the two sides may be forced to make some compromises.

“Both parties should realise that engaging in intense confrontation is obviously not the way forward,” said Piotr Trudnowski, a member of Klub Jagiellonski, a Christian-Democrat think tank.

- ‘Poland First, Poles First’ -

Ewa Marciniak, a political scientist at the University of Warsaw, said that Nawrocki would have to work with the government on foreign policy – in which he has “no experience”.

During the election campaign, he highlighted the importance of ties with the United States and his close ties with Trump.

“It is precisely from this that he will build his foreign policy, at least initially,” Marciniak said.

One major difference could be relations with Ukraine.

Trudnowski said that Nawrocki would “not be as enthusiastic” as his predecessor Andrzej Duda on Ukraine.

During his campaign, Nawrocki opposed the idea of NATO membership for Ukraine and criticised Kyiv for not having “shown gratitude for what the Poles have done”.

Under his slogan “Poland First, Poles First”, he was critical of some of the benefits received by the more than one million Ukrainians who have fled to the country.

Ukrainian President Volodymyr Zelensky congratulated Nawrocki on Wednesday and said he looked forward to “constructive and direct dialogue”.