Independent hard-right candidate Călin Georgescu, 62, who had been polling in single digits before the election, shocked observers by winning the first round on Sunday, the Reuters news agency reported.
His success has raised concerns about potential shifts in Bucharest's staunch pro-NATO and pro-Ukraine stance.
The leaders of Romania's ruling coalition parties—the leftist Social Democrats (PSD) and the centre-right Liberals (PNL)—both failed to advance to the second round.
This unprecedented outcome reflects voters' rejection of establishment politics, Reuters reported.
In response, both party leaders resigned.
Ilie Bolojan, the newly appointed head of the Liberal Party, announced late on Monday that the party would back Elena Lasconi, a centre-right, pro-European candidate and leader of the opposition Save Romania Union (USR), in the decisive run-off.
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Source: IAR, PAP, Reuters
Click on the audio player above for a report by Radio Poland's Michał Owczarek.