South Korea's main opposition party looks set to win at least 11 of 16 posts in key local and regional elections, according to exit polls.
Park Geun-hye is a leading presidential contender
The votes are being seen as a key test for the government of President Roh Moo-hyun, increasingly unpopular as it approaches its final year in office.
The ruling Uri party has secured just one post, the exit polls suggested.
The election could also have far reaching consequences for the South's relations with North Korea.
The opposition Grand National Party (GNP) was dominating the races for mayors, governors and regional assembly members, KBS and MBC TV exit polls suggested.
"It appears we have failed to read the people's minds," Uri Party campaign chairman Yum Dong-yun told KBS TV.
Roh under fire
Mr Roh's government is accused by its opponents of being incompetent and divisive in its running of the economy and in foreign affairs.
The government has irritated the United States with what is seen as an indulgent policy towards North Korea, despite the North's claim to be developing nuclear weapons.
The GNP has also gained public sympathy as a result of a knife attack by an unemployed ex-prisoner on party leader Park Geun-hye during election campaigning 10 days ago.
She received an 11-cm (four-inch) cut to her cheek which required 60 stitches.
A big win for the opposition would strengthen its position in the run-up to presidential elections next year.
It may also boost the chances of Ms Park, daughter of former military President Park Chung-hee, of winning the presidency.
President Roh is prevented by law from seeking re-election.
The GNP has been pushing for a tougher line on aid to the North and for improved relations with the US.
North Korea has warned about a rise in what it sees as hostile political forces in the South.
It recently cancelled the test run of a train across the heavily fortified border, citing, among other reasons, political instability in South Korea.