Voters headed to polling stations Wednesday for the crucial parliamentary elections that will shape the legislative landscape for the next four years and gauge public sentiment ahead of next year's presidential race.

The quadrennial polls are to elect a new 300-member National Assembly and take on extra significance as the results will affect how President Park Geun-hye wraps up her remaining term in office, as well as set the tone for who will succeed her.

The National Assembly will be comprised of 253 directly contested seats and 47 proportional representation seats to be allocated to parties according to the total number of votes they receive. Each voter is asked to cast two ballots: one for a candidate and the other for a party.

Voting opened at 6 a.m. and will close 12 hours later.

Voter turnout stood at 16.1 percent as of 11:00 a.m., with 6.79 million of the total 42.1 million eligible voters having cast their ballots, the National Election Commission said.

The turnout rate is lower than that recorded in the previous regional elections in 2014, when the number reached 18.9 percent at the same point in time.

The NEC expects the final voter turnout may reach some 60 percent although full official tallies are not expected until about 2:00 a.m. Thursday.

The competition is basically a two-way race between the ruling Saenuri Party and the main opposition Minjoo Party of Korea, with Saenuri forecast to win an easy victory, although it may lose some seats in its traditional strongholds to independents.

However, many also forecast that the splinter People's Party will grab more than 20 seats, which would allow it to form a parliamentary negotiation bloc.

Voter turnout will affect the outcome of the polls as a higher turnout would mean more young voters participated in the balloting.

The opposition parties are traditionally more popular among younger voters, while older voters are more supportive of the conservative ruling party.

The surveys conducted by four local pollsters last week showed that Saenuri could grab between 150 to 175 seats, while the Minjoo Party of Korea will likely secure fewer than 100 seats, shy of its initial goal of 107 seats.

The People's Party is likely to win 28-32 seats on the strong backing of voters in the Honam region in the southeastern part of the country, according to the surveys.

A majority win for the ruling party will allow Park to exercise control over state affairs more effectively and make it possible for her to push for various economic and social reforms.

Wednesday's elections are also a crucial test for potential presidential hopefuls, including Kim Moo-sung, head of the ruling Saenuri Party; Moon Jae-in, former leader of the main opposition Minjoo Party of Korea; and Ahn Cheol-soo, who left the Minjoo Party to create the People's Party.

South Koreans will go to the polls in December 2017 to elect a new president to succeed Park, whose single five-year term ends in early 2018. By law, she cannot seek re-election.

Kim, who is seeking his sixth parliamentary term in the southern port city of Busan, has a lot to prove to party members although the party's victory could help smooth over the recent feud between various factions.

Moon's election victory would also solidify his image as a competent presidential candidate, while Ahn's successful achievement of taking more than 20 seats would prove his credentials as a potential leader.

Eligible voters in Wednesday's polls total about 42.1 million, more than 80 percent of South Korea's 50 million population, the NEC said.

Voters will cast their ballots at a total of 13,837 polling stations across the country, it said. (Yonhap)