By Christo Johnson and Katrina Manson

FREETOWN, Aug 17 (Reuters) - Sierra Leone's main opposition candidate, Ernest Bai Koroma, is leading the race for the presidency with almost two thirds of votes counted from last weekend's poll but a second-round runoff looks likely.

Koroma was in the lead with 45 percent while the country's vice president, Solomon Berewa -- candidate for the ruling Sierra Leone People's Party (SLPP) -- had polled 37 percent, with 62 percent of votes counted, the electoral commission said.

A runoff is due to be held in September if no candidate wins 55 percent in the first round.

"I am definitely disappointed with the results from the western area and some other parts of the country," Berewa told Reuters in the capital Freetown.

"But I am still with the hope that Ernest Bai Koroma and I will go to a runoff when the results are officially announced."

Results in so far showed Koroma performing strongly in the north and west of the former British colony as well as taking more votes than expected in some eastern and southern areas such as Kono, Bo and Kenema, traditionally SLPP territory.

Berewa's share of the vote has been reduced by the PMDC, a breakaway group which split off from the SLPP in 2006 and whose leader, Charles Margai, was on 15 percent.

The opposition All People's Congress (APC) said on Wednesday it had won a parliamentary majority in legislative polls held alongside the presidential vote last Saturday, based on counting by party officials, and vowed to mount a legal challenge if official results still trickling in showed otherwise.

Expectations for change are high in a country second from bottom of the U.N. Human Development Index where unemployment is at about 60 percent, most people survive on under $1 a day, and the state depends on foreign aid for a third of its budget.

The polls were the first since U.N. peacekeepers left the West African country two years ago and are seen as a test of its ability to recover from a 1991-2002 civil war, one of the most brutal in modern African history.

Many Sierra Leoneans say they are tired of the SLPP's failure to tackle rampant corruption, which analysts and diplomats say means badly-needed aid is being squandered.

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