Il Nuovo Postiglione - Pro-EU centrist Nicusor Dan sworn in as Romania president

A week ago, Dan won a tense election rerun, seen as key for the direction of the NATO and European Union country bordering war-torn Ukraine.

Although EU critic and nationalist George Simion easily topped the first round of voting, Dan took victory in the second round run-off.

The election came five months after Romania's constitutional court scrapped a presidential ballot over allegations of Russian interference and the massive social media promotion of the far-right frontrunner, who was not allowed to stand again.

Dan, 55 and Bucharest mayor since 2020, took the oath of office in parliament.

"The Romanian state needs a fundamental change, within the rule of law, and I invite you to continue to be involved in order to put positive pressure on state institutions to reform," Dan told the assembly after being sworn in.

"I call on political parties to act in the national interest."

Lawmakers from Simion's far-right AUR party did not participate in the ceremony, saying it was "legitimising a national treason".

Later on Monday, Dan was to meet interim president Ilie Bolojan -- who has been tipped as a possible prime minister.

Dan has vowed to usher in a "new chapter" in Romania.

He travelled to Warsaw on Sunday to support pro-EU Mayor Rafal Trzaskowski, who hopes to emerge the winner of Sunday's race for the Polish presidency.

"We won the Romanian presidential elections. People rejected isolationism and Russian influence," Dan told thousands of people at a really for Trzaskowski.

- 'We will resist' -

Tensions remained high in Romania, with authorities on alert after hundreds of TikTok users called for protests on Monday.

"We will resist!" Simion, 38, wrote on Facebook on Sunday, although he specified that he would not be protesting on Monday.

Romania's constitutional court validated the election results on Thursday, after rejecting an appeal by Simion to annul the vote.

Simion has repeatedly alleged that there was foreign interference in the vote, including by France, and electoral fraud, without giving any evidence.

He condemned the court's decision as a "coup d'etat".

Simion comfortably topped the first-round ballot on May 4, campaigning against what he called the EU's "absurd policies" and vowing to cut aid to Ukraine.

But Dan won almost 54 percent of the run-off vote.

Romania's president has significant sway in foreign policy -- including representing the country at NATO and EU summits -- and appoints key posts, such as the prime minister.

Dan, a mathematics prodigy who has promised to fight corruption for an "honest" Romania, has said he would speak to all four pro-Western parliamentary parties about "not only the appointment of the prime minister but also the outline of the government's programme", including judicial reforms.

Romania, a country of 19 million people, is struggling with high inflation. It is the EU's most indebted country.

Dan, who is known for his reserved nature, has voiced support for Ukraine and vowed to keep Romania on its "pro-Western" path.

O.Nardolillo--INP