South Koreans are voting in closely-contested parliamentary elections, seen as a key test for President Lee Myung-bak.

Polls opened early Wednesday local time and will close 12 hours later. Voters are casting ballots to elect a new 300-member National Assembly.

Most of the legislators will be elected directly, with the rest chosen through proportional representation.

The results will have an impact on the December presidential vote. This is the first time in 20 years that the parliamentary and presidential election are taking place in the same year.

The competition is largely a two-way race between the ruling New Frontier Party and the opposition Democratic United Party.

Currently, the conservative ruling party has the majority in parliament, but polls show the party may not retain it. The opposition has accused the party of supporting the rich and privileged at the expense of ordinary South Koreans.

Elections for South Korea's 300-seat unicameral parliament are held every four years. The country has about 40 million eligible voters.