Hard-right populist George Simion will contest Romania's presidential election result.
The 38-year-old had been the favourite to win at the polls - but unexpectedly lost to pro-Western candidate Nicusor Dan over the weekend.
Mr Simion now claims to have hard evidence of foreign interference in the vote.
He said in a statement: "We now have irrefutable evidence of meddling by France, Moldova, and other actors, in an orchestrated effort to manipulate institutions, direct media narratives, and ultimately impose a result that does not reflect the sovereign will of the Romanian people."
In the first round, the Trump supporter took 40.96% of the votes, putting him almost 20 points ahead. But the second round saw Mr Dan, the mayor of Bucharest, end up with 54% of the votes, while Mr Simion secured 46%.
A voter turnout of 64.7% in Sunday's ballot, more than in any Romanian election of the past 25 years, is thought to have benefited Mr Dan.
Mr Simion has submitted a formal request to Romania's Constitutional Court, calling for it to annul the result.
Mr Dan, a 55-year-old mathematician, ran as an independent candidate and his pledges included a clampdown on rampant corruption and maintaining support for Ukraine.
"We need to build Romania together irrespective of who you voted for," he said when his victory was assured.
Romania and its judiciary now face a difficult choice
It's three days since I asked George Simion if he would accept the result if he lost, and he said "yes" with the sort of shrug that suggested it was a stupid question.
Turns out it was quite a good question, after all.
Because Mr Simion has just stated that he won't accept the result, after all.
He's alleging the French government tried to limit the amount of his campaign material that appeared on social media, echoing an accusation made by Pavel Durov, the Russian founder of the messaging app Telegram.
Durov claimed "a Western government" had asked his company "to silence conservative voices in Romania" ahead of the election.
He added an emoji of a baguette onto his message as a not-too-subtle clue to the government he meant.
Durov is enmeshed in a legal row with French authorities. He was arrested by French police in August 2024, facing the allegation that a lack of content moderation on Telegram had allowed criminal activities.
He was forced to remain in France until two months ago, when he was allowed to return to his home in Dubai.
Simion has now told his followers to only use Telegram, adopting that to be their only form of communication.
What this means for Romania is more turmoil and more rancour.
Nicusor Dan is due to be installed as president just at a time when Simion is encouraging his millions of supporters to deluge the country's highest court with demands that the election be run again.
But the country, and its judiciary, face a difficult choice.
They annulled the December election on the basis of evidence that even Georgescu's opponents thought was questionable.
Can they really now ignore Simion's claims and press on regardless without accusations that they favour the mainstream politicians over the populists?
And that, of course, would hugely fuel Simion's long-running accusation that the establishment is out to thwart him.
It is, in short, a bit of a mess.

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After exit polls indicated the pro-EU candidate had won, Mr Simion posted on Facebook: "I won!!! I am the new President of Romania and I am giving back the power to the Romanians!" However, he conceded the election later in the evening.
Mr Simion - a nationalist banned from Ukraine, who has advocated for uniting Romania with Moldova - surged to prominence after the country's first attempt to hold its presidential election last year.
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Far-right outsider Calin Georgescu topped the first-round polls, but Romania's highest court disqualified him and annulled the result after allegations of electoral violations and Russian interference.
Mr Simion capitalised on the furore, offering his support to Mr Georgescu and promising to appoint him as prime minister if he secured the presidency.