The top two candidates will face each other in the second round of the ballot on December 8.
Exit polls showed Ciolacu with 25% and Lasconi with 18% share of the vote, while Călin Georgescu and George Simion, the two hard-right candidates, were at 16% and 15% of the vote.
The data does not include the votes of hundreds of thousands of Romanians who live abroad, who can still influence the result. Both Simion and Lasconi are popular among Romanians abroad.
Analysts had predicted that social democrat Ciolacu would win a second round against Simion, who had been his main contender in opinion polls ahead of the vote. Ciolacu has touted his experience running Romania during a war in neighboring Ukraine. However, they say he may struggle against Lasconi.