Parvanov won 64 per cent of ballots cast in Sunday's elections while Volen Siderov, his nearest rival who heads the anti-EU Attack party, won 21.5 per cent, official results showed on Monday.
But with a voter turnout of 42.1 per cent, below the legally required minimum of 50 per cent, the pair must now face each other on October 29.
Parvanov, former head of the ex-communist Socialist party, has won widespread support in the Black Sea country of nearly eight million people for raising Bulgaria's profile abroad on the way to joining the Nato alliance in 2004 and the EU on January 1.
His main opponent, the nationalist Volen Siderov, is known for his hard-line policies against Bulgaria's minorities - especially gypsies and Turks.
The third most popular candidate, Nedelcho Beronov, is from Bulgaria's fragmented right-of-centre opposition and is the former head of the country's constitutional court.
Siderov is known for his strong stance against minorities
Although he casts himself as non-partisan, experts say if Parvanov is re-elected it will help consolidate power for his ruling socialist allies as the Black Sea country joins the EU.
Analysts say he has used his limited powers, which include the ability to veto laws and appoint cabinets, to become one of the country's most influential figures.
His critics say he has failed to push successive cabinets to stop high-level corruption and organised crime and raise living standards that - with average wages of 160 euros ($202) a month - will be the EU's lowest.