South Korean President Park Geun-hye

Seoul - South Koreans voted on Wednesday in regional and mayoral elections seen as a referendum on embattled President Park Geun-hye and her government's handling of a ferry disaster that killed more than 300 people in April.

The sinking of the ferry Sewol has sidelined traditional campaign issues, such as jobs, education and welfare, and focused attention on a debate over the perceived failure of Asia's fourth-largest economy to enforce safety standards.

“Because we must not forget the Sewol disaster, I believe this incompetent national (government) must be brought to justice through this election,” opposition leader Ahn Cheol-soo said on Tuesday before campaigning ended at midnight.

Park's approval ratings fell sharply from 61 to 46 percent in the days after the April 16 disaster - her lowest since coming to power in February 2013 - and show little sign of improving.

Opposition candidates look set to win key mayoral races in Seoul, neighbouring Incheon and Busan in the south-east, but Park's ruling conservative Saenuri Party is unlikely to suffer landslide defeats.

Polls by Gallup Korea in late May suggested more than 40 percent of voters still support the Saenuri Party. Only 28 percent said they supported the main opposition New Politics Alliance for Democracy (NPAD).

About 40 percent of voters in South Korea are older than 50 and traditionally vote conservative - unlike younger voters, who are less likely to cast ballots.

The opposition has also been careful not to use the government's handling of the disaster as a political tool when distrust of politicians across the board has increased.

The elections are the first nationwide polls since Park took office.

Liberal Seoul mayor Park Won-soon, of the NPAD, has a 10 percent lead over the conservative candidate and is expected to hold on to his post, seen by some as a springboard to the presidency.

The Sewol ferry sank on a routine trip south from the port of Incheon to the holiday island of Jeju on April 16. More than 300 people, most of them students from the Danwon High School on the outskirts of Seoul, were killed.

The ferry was heavily overloaded, was travelling too fast on a turn and many of its crew abandoned ship as the children waited in their cabins as they had been instructed to do. - Reuters