The tight finish - with fewer than 1.5% of the ballots still to be counted - is far from a resounding endorsement of the pro-EU path that Sandu has pursued over four years at the helm of the small ex-Soviet republic tugged between Russia and the West.

A presidential election, which took place simultaneously, handed Sandu 42% of the vote while her main rival, former prosecutor-general Alexandr Stoianoglo won 26%, setting up a tightly fought run-off between the two on Nov. 3.

The votes, which took place after a slew of allegations of election meddling, were seen as a test of the southeast European nation's commitment to join the European Union and escape Moscow's orbit for good.

The tight referendum result puts Sandu in a weaker position going into the second round since she has championed EU integration.

Moldova began the long process of formal accession talks in June and under Sandu has aimed to join by 2030. Ties with Moscow have deteriorated as Sandu condemned the Kremlin's invasion of Ukraine and diversified energy supply away from Russia.

Stoianoglo has said that, if he comes to power, he would develop a "balanced" foreign policy involving ties with the EU, Russia, United States and China. He boycotted Sunday's referendum, calling it a ruse to boost Sandu's haul at the election.

'CLEAR EVIDENCE'

In the early hours of Monday, Sandu addressed Moldovan citizens, saying there was "clear evidence" that criminal groups working together with "foreign forces hostile to our national interests" had sought to buy off 300,000 votes.

She said this amounted to "fraud of unprecedented scale".

"Criminal groups... have attacked our country with tens of millions of euros, lies and propaganda, using the most disgraceful means to keep our citizens and our nation trapped in uncertainty and instability," she said.

While the still waiting for the final results, she said, Moldova would "respond with firm decisions", without elaborating.

In the run-up to the vote, authorities made repeated statements alleging concerted attempts to meddle in the vote by fugitive tycoon Ilan Shor, who lives in Russia.

Russia, which accuses Sandu's government of "Russophobia", denied interfering, while Shor denies wrongdoing.

The police accused Shor, who was sentenced to jail in absentia for fraud and a role in the theft of $1 billion, of trying to pay off a network of at least 130,000 voters to vote "no" and support "our candidate" at the election.

Shor has openly offered on social media to pay Moldovans to convince others to vote in a certain way and said that was a legitimate use of money that he earned.

In the early hours of Monday, he declared Moldovans had voted against the referendum.

"Today I congratulate you, you lost the battle," he added, addressing Sandu simply as Maia.

Ahead of the vote, authorities took down online resources they said hosted disinformation, announced they had uncovered a programme in Russia to train Moldovans to stage mass unrest and opened criminal cases against allies of Shor.

As the early results came in late on Sunday, some 57% of Moldovans initially appeared to have voted "no" in the referendum. As more ballots were counted, the "yes" vote gradually rose, overtaking "no" early on Monday morning.

Political analyst Valeriu Pasha said the "yes" vote had edged ahead only because of unusually high voter turnout among the Moldovan diaspora living abroad, who largely support EU integration.

"With such elections, in which dozens of (percentage points) can be bought, it will be very difficult for us going forward. But we must learn lessons and learn to fight this phenomenon," he said.