Russia had already denied meddling in the vote, which came a week after another key Eastern European election in Georgia, whose president said it had been a “Russian special operation”.
Alexandr Stoianoglo, who was fired as prosecutor general by Sandu, has denied being pro-Kremlin.
Voting ended in Moldova at 21:00 (19:00 GMT), with a 54% turnout higher than four years ago, and especially high among expat voters at polling stations abroad.
The result is likely to become even tighter as results from polling stations in the EU and the US become clear. The final result will be declared on Monday.
As polls closed, both Maia Sandu and her rival thanked voters, with Stoianoglu speaking in Russian as well as Romanian. Although Romanian is Moldova’s main language, Russian is widely spoken because of its Soviet past.
Casting his ballot, he promised to be an “apolitical president” for everyone, telling reporters he had voted for “a Moldova that should develop in harmony with both the West and the East”.
Stoianoglu polled particularly well in rural areas and the south, while Sandu was ahead in the cities and abroad, Moldovan media reported.