The pro-European Union candidate in Romania's critical presidential run-off has won the closely watched race against a hard-right nationalist, nearly complete electoral data shows, in a tense election rerun that many viewed as a geopolitical choice between East and West.

The race pitted front-runner George Simion, the 38-year-old leader of the hard-right Alliance for the Unity of Romanians, or AUR, against Nicusor Dan, the mayor of Bucharest.

It was held months after the cancellation of the previous election plunged Romania into its worst political crisis in decades.

After 10.5 million of 11.6 million votes had been counted, Dan was ahead with 54.32 per cent, while votes for Simion stood at 45.68 per cent, according to official data.