The main opposition Democratic United Party (DUP) on Monday selected five finalists from eight contenders for its presidential primary in an effort to increase the competitiveness of its candidates.

Three less popular candidates were eliminated through two days of telephone surveys of thousands of ordinary citizens and registered party members, which ended at 10 p.m. Monday.

The five chosen include major candidates Moon Jae-in, a former chief of staff to late President Roh Moo-hyun, Sohn Hak-kyu, a special advisor for the DUP, former South Gyeongsang Province Governor Kim Doo-gwan, and Rep. Chung Sye-kyun, a three-term lawmaker who served as the party's chief, the party announced on Tuesday.

Park Joon-yung, governor of South Jeolla Province, won the last ticket for the primary, according to the party.

The party, however, did not unveil each contender's ranking and share of votes, citing potential influence on the primary.

Moon is widely seen as a serious opposition contender to Park Geun-hye, the leading presidential hopeful of the ruling Saenuri Party and a front-runner in opinion polls for the December presidential election.

Recent polls, however, showed that Moon was behind Park and Ahn Cheol-soo, a popular software entrepreneur-turned-professor who has not yet announced his presidential bid.

The DUP said it plans to designate its presidential nominee on Sept. 16. A runoff will be held on Sept. 23 if none of the five contenders win a majority of the votes in primary elections that are open to registered voters and ordinary citizens, according to the party.

The DUP's predecessor lost the presidential election in 2007 after a decade of liberal rule in South Korea.

South Korean President Lee Myung-bak's single five-year term ends in February and by law, he cannot seek re-election. (Yonhap News)