Second round decider appears nearly certain as the threshold for a first-round win is 55%.
The opposition challenger in Sierra Leone's presidential election edged ahead of the ruling party candidate by fewer than 15,000 votes, according to new partial results released on Sunday.
Julius Maada Bio has garnered 43.4 per cent of the vote to 42.6 per cent for Samura Kamara with 75 per cent of polling stations in each province reporting, the National Election Commission (NEC) announced.
The National Grand Coalition headed by former UN diplomat Kandeh Yumkella, which is hoping to inflict damage on the two-party system, stood at 6.69 per cent, the NEC said.
Full results are expected in the next few days, but a second round decider appears nearly certain as the threshold for a first-round win is 55 per cent.
President Ernest Bai Koroma, who cannot run again after consecutive five-year terms, anointed Kamara as his successor for the ruling All Peoples Party (APC).
The APC and Bio's Sierra Leone Peoples Party have alternately ruled Sierra Leone since independence from Britain in 1961.
European Union observers on Friday described the election as "transparent, credible and well-organised" on the whole.
But they noted sporadic "intimidation and violence," concerns echoed by the Freetown-based Campaign for Human Rights and Development International (CHRDI).
Early results triggered unrest as supporters of the two main parties clashed in Freetown's central business district, leading to several arrests.
A runoff poll must take place two weeks after final results are unveiled.
The export-dependent economy of the mineral-rich but impoverished West African country is in a dire state following the 2014-16 Ebola crisis. A commodity price slump has driven away foreign investors.