SEOUL (Reuters) - South Korea's liberal opposition picked a human rights lawyer as its presidential candidate on Sunday in a bid to derail conservative frontrunner Park Geun-hye's ascent to the top job in December's polls.

Moon easily beat three minor contenders from the Democratic United Party (DUP) in the three-week party primaries but faces a formidable task of trying to form an alliance with the independent and popular software entrepreneur Ahn Cheol-soo if the left of centre is to make a serious challenge against Park.

"You have chosen a change for the Republic of (South) Korea, a transfer of government power, a victory of the Democratic United Party and me," he said in his acceptance speech before the cheering audience at the party convention.

Moon, an earnest but lacklustre speaker whose most prominent job was chief of staff to former President Roh Moo-hyun, has tried to paint Park as little more than a privileged child of a former president while he grew up in poverty.

Moon, 59, trails a long way behind Park in public opinion polls and has consistently ranked third behind Ahn, who has no party affiliation.

Park, daughter of slain leader Park Chung-hee, is the clear favourite to win the December 19 poll to elect the leader for a single five-year term at a time when the export-driven economy struggles to maintain the pace of growth.

Moon, who was jailed for taking part in democracy protests in 1975 during Park's father's presidency, is a relative unknown in the national political scene.

Ahn, who shot to the forefront of the political scene a year ago as a potential candidate for the mayor of Seoul, has said he would make public his decision whether to stand for the presidency after the end of the Democratic primaries.

Moon did not rule out forming an alliance with Ahn, saying on YTN TV that whoever stands as the coalition candidate should represent the Democratic United Party.

(Additional reporting by Ju-min Park and Sung-won Shim; Editing by Nick Macfie)

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