By Richard Valdmanis and Alphonso Toweh

MONROVIA (Reuters) - Liberia is to release a first batch of official results on Thursday from its hotly contested presidential election, in which Nobel laureate Ellen Johnson-Sirleaf is seeking a second term.

An unofficial partial tally released on Thursday by a local media association, the Liberia Media Center (LMC), showed Johnson-Sirleaf had an edge over her main rival Winston Tubman, with 44 percent to his 36 percent of some 380,000 votes counted.

Ex-rebel leader Prince Johnson is in third place with 12 percent, according to the LMC figures, which it says are a tally of results posted by the National Election Commission at individual polling stations around the country. That could make him a possible kingmaker in any second-round runoff.

"If there is a run-off, I will get to my constituencies to ask them which way to go. Based on what they will tell me, I will then make a decision, but for now, I cannot say anything. We represent a huge group of people," Prince Johnson told Reuters on Thursday.

The vote is seen as a test of Liberia's progress since the 1989-2003 civil war killed nearly a quarter of a million people and left infrastructure in ruins. If smooth, the election could pave the way to billions of dollars in investment in Liberia's mining, energy and agriculture sectors.

"We are all waiting for the results, and from my perspective, I think they will be accepted," said Amadou Kante, a resident of the Sinkor neighborhood in the capital Monrovia.

Around 1.8 million Liberians registered for Tuesday's election, the second since the fighting and the first to be organized locally. If no candidate wins an outright majority, the two front-runners from a field of 16 will go into a run-off vote scheduled for early November.

Official preliminary figures will be released by the National Election Commission later on Thursday. Many voters were tuned to radio or television stations as the LMC's unofficial tally came through.

"I think for now, the election process was calm and from what we are looking at, there may be a second round. But this is elections, anything can happen," said Omeja Jimmie, a student of the African Methodist University in Monrovia.

Voting on Tuesday passed off peacefully in Monrovia. Observer groups said they had received no reports of trouble elsewhere in the country of four million people, but have expressed concern that the results could be a flashpoint.

"The mission is of the view that there were no major irregularities and incidents of violence. It estimated that on the whole, the elections of October 11, 2011 were conducted under acceptable conditions of freedom of voters and transparency of the process," said Attahiru Jega, head of the observer mission from West African bloc ECOWAS.

Johnson-Sirleaf got a pre-poll boost with her award of the Nobel Peace Prize last Friday, but rivals have said Liberians will judge her on her success in fighting poverty in a country with an average annual income of $300 a head.

A dispute over the results of the 2005 election that brought Johnson-Sirleaf to power as Africa's first freely elected female head of state triggered several days of rioting.

(Writing by Richard Valdmanis; Editing by Giles Elgood)