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.
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 percent of the vote to 42.6 percent for Samura Kamara with 75 percent 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 percent, 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 percent.