Polls opened Wednesday in South Korea's closely contested parliamentary elections, seen as a key test of sentiment ahead of December's presidential poll.
Voting began at 6:00 am (2100 GMT) at 13,470 polling stations across the country with 40 million eligible voters, a National Election Commission spokesman told AFP.
Polls will close at 6:00 pm (0900 GMT), with official results expected past midnight.
The ruling conservative New Frontier Party (NFP) is struggling to preserve its parliamentary majority in the election, a prelude to what it hopes will be a second successive presidential victory.
NFP had 165 seats in the outgoing parliament against 89 for the main opposition centre-left Democratic United Party (DUP), but Wednesday's vote is seen as too close to call.
Opinion polls are banned this week but experts predict a dead heat, with each side winning 130-135 seats in the 300-member National Assembly.
Voter turnout is closely watched. A high turnout, especially among young voters, is seen as advantageous to the opposition, centre-left party.
The opposition has attacked the government of President Lee Myung-Bak, blaming it for difficulties such as inflation and a widening income gap.
The ruling party has ditched its old name of the Grand National Party and moved to the left in a bid to try to shake off its image as a party for the rich, and has promised to improve state welfare programmes.
The NFP backs a recently ratified free trade agreement with the United States, while the DUP has vowed to renegotiate it.
The ruling party has depicted the DUP as socially divisive and bent on undermining a decades-old security alliance with the United States.
Despite North Korea's plan to launch a rocket in the coming days, more everyday concerns such as the economy and welfare state have dominated the election.
Both parties pledge to expand state welfare and tighten controls over the mighty conglomerates, or chaebol, which dominate the economy and are accused of stifling small business.
Presidents serve a single five-year term and parliaments are elected for four years. This will be the first time for two decades that the votes fall in the same year.