Romania's top court has annulled the result of the first round of the country's presidential election, adding that the entire election process will have to be re-run.
The second round had been due to be held on Sunday and voting is already underway in polling stations abroad.
It would have pitted nationalist independent Calin Georgescu against pro-European Union reformist conservative Elena Lasconi.
Having polled in single digits before the first presidential election round on November 24, Georgescu - who wants to end Romanian military support for Ukraine against Russia's invasion - surged to a first-place finish that raised questions over how such a surprise had been possible.
A Georgescu win would have upended the EU and NATO member state's politics, pushing it closer to a belt of states in central and eastern Europe with powerful populist, Russia-friendly politicians including Hungary, Slovakia and Austria.
However, Friday's ruling plunged the country into institutional chaos as current President Klaus Iohannis's term ends on December 21 and it was unclear who would be head of state after this date.
Documents declassified by Romania's top security council on Wednesday said the country was a target of "aggressive hybrid Russian attacks" during the election period.
"The electoral process to elect Romania's president will be fully re-run, and the government will set a new date and ... calendar for the necessary steps," the court said in a statement.
It added the ruling was made "seeking to ensure the fairness and legality of the electoral process".
A detailed explanation of its ruling will be released at a later date.
The court had validated the first presidential round on Monday.
Georgescu said on Friday that the court ruling was a "coup," according to a written statement given to broadcaster Realitatea TV.
Lasconi also condemned the ruling and said the vote should have kept going, respecting the will of Romanian people.
"The constitutional court's decision is illegal, amoral and crushes the very essence of democracy, voting," she said.
"We should have moved forward with the vote. We should have respected the will of the Romanian people. Whether we like it or not, from a legal and legitimate standpoint, nine million Romanian citizens, both in the country and the diaspora, expressed their preference for a particular candidate through their votes. We cannot ignore their will!" she said.
"I know I would have won. And I will win because the Romanian people know I will fight for them, that I will unite them for a better Romania. I will defend our democracy. I will not give up."
Social Democrat Prime Minister Marcel Ciolacu supported the move, calling it "the only correct solution" after declassified documents pointed to Russian interference.
Populist conservative parties also performed well in last Sunday's parliamentary election in Romania, although the ruling Social Democrats emerged as the largest grouping and hope to cobble together a pro-EU coalition government.
The court has not called into question the integrity of the parliamentary vote.
George Simion, the leader of the opposition hard-right Alliance for Uniting Romanians called the court ruling a "coup d'etat," adding "nine politically appointed judges, scared that a candidate outside the system had all chances to become Romania's president, decided to annul Romanians' will".
In one of the declassified documents, Romania's intelligence agency said Georgescu was massively promoted on social media platform TikTok through co-ordinated accounts, recommendation algorithms and paid promotion.
Georgescu has declared zero funds spent in the campaign.
Russia has denied any interference in Romania's election campaigns.
TikTok denies giving Georgescu special treatment, saying his account was labelled as a political account and treated like any other.
It was not yet clear if Georgescu would be allowed to take part in the re-run election.
with AP
