Igor Dodon needed at least 51% to avoid a second round on 13 November.

Sunday's poll was the first direct presidential election in Moldova for 20 years.

It is seen as a battle between those supporting closer ties with Russia and those wanting integration with the European Union.

With turnout put at about 48%, Maia Sandu said young people had been "impeded" from voting, but gave no details.

"Today the young had a small turnout. I think it happened because the authorities impeded them from doing so. We will do everything to remove such obstacles in the second round," she said.

Mr Dodon thanked his supporters, adding: "The main conclusion is that voters no longer believe in this government. Our victory is inevitable."

Prime Minister Pavel Filip said in a statement that he hoped the result of the run-off would "bring about both change and stability".

The ex-Soviet republic is one of the poorest countries in Europe and has been hit by a string of high-profile corruption scandals.

Since 1996 presidents of Moldova have been chosen by parliament.

Both the EU and Russia seek greater influence over the country, which is wedged between Ukraine and Romania, but polls indicate that voters are divided about which offers them a better future.

Mr Dodon has pledged to push for new parliamentary elections if he wins. Ms Sandu, from the centre-right opposition, has been praised for reforming the education system.

The president appoints judges and sets out foreign policy but other major decisions need the approval of parliament.