In a joint statement congratulating Sandu on her re-election, the European Commission and the EU’s top diplomat Josep Borrell said there had been “unprecedented interference by Russia”.
As polls closed, both Sandu, 52, and her rival thanked voters, with Stoianoglo speaking in Russian as well as Romanian. Although Romanian is Moldova’s main language, Russian is widely spoken because of its Soviet past.
Turnout at 54% was high, especially among expat voters at polling stations abroad.
Stoianoglo took an initial lead on the night and was the more successful candidate in Moldova itself with more than 51% of the vote. Sandu won in the capital Chisinau, and she was completely dominant among expat voters.
As she overtook her challenger late on Sunday night, there was cheering at her campaign headquarters and chants of “victory”.
In a hoarse voice she praised her compatriots for saving Moldova and giving “a lesson in democracy, worthy of being written in history books”.
Then, moving into Russian, she said: “I have heard your voice – both those who supported me and those who voted for Mr. Stoianoglo. In our choice for a dignified future, no-one lost… we need to stand united.”
Maia Sandu’s foreign policy adviser, Olga Rosca, told the BBC she was proud of the result.
Asked whether she was surprised that Stoianoglo had won in Moldova itself, she said the vote in Moldova and abroad should be seen as one and the same: “We never divide people into Moldovans at home and expatriates – we see Moldovans as one family.”
With elections coming next year she said the president had “clearly indicated she has heard the mood for change. On several occasions between the [two presidential] votes she said the fight against corruption must be intensified and justice reform must be accelerated – she’s committed to this work”.
The final result will be declared on Monday.
EU Commission President Ursula von der Leyen congratulated Sandu, saying “it takes a rare kind of strength to overcome the challenges you’ve faced in this election.
“I’m glad to continue working with you towards a European future for Moldova and its people,” her message on X said.
Casting his ballot, Alexandr Stoianoglo had promised to be an “apolitical president”, and that he had voted for “a Moldova that should develop in harmony with both the West and the East”.
Stoianoglo polled particularly well in rural areas and the south, while Sandu was ahead in the cities and with young voters.