South Koreans voted Wednesday in local elections seen as a spot referendum on President Park Geun-Hye's handling of the April ferry disaster that killed about 300 people, mostly schoolchildren.

Park's administration has been sharply criticised for its response to the tragedy and the polls were the first real opportunity to measure the severity of the political fallout on a national level.

The high popularity ratings Park has enjoyed since taking in office in February 2013 have been hammered by the disaster, which has become the defining moment of her presidency so far.

The investigation into the sinking of the 6,825-tonne Sewol on April 16 exposed a culture of institutional negligence, greed and incompetence that contributed to the scale of the tragedy.

Although these problems have roots stretching back decades, Park and her ruling conservative Saenuri Party have become a default focus for much of the public grief and anti-establishment anger.

As a result, Wednesday's elections for 17 new provincial governors and municipal mayors, as well as numerous local councils, are being seen as a snap vote of confidence in Park's leadership.

"Other issues have largely been swept aside and the real question has become about defending Park or holding her to account," said Choi Jin, head of the Institute of Presidential Leadership.

Among voters in Seoul, that choice was clearly uppermost in many people's minds.

"I really believe the ruling party should be punished for what they did, or rather didn't do, over the Sewol," said June Kim, a 35-year-old office worker.

Tragedy 'front and centre'

"They failed miserably ... and they deserve to be spanked for it," Kim said.

An older voter said she had chosen all Saenuri candidates as a show of support for Park Geun-Hye.

"I think the president and the other officials did the best they could with the ferry accident," said the 57-year-old, surnamed Nam.

Politics in South Korea, and especially local politics, have a strong regional bias, with entrenched support for certain parties that rules out any seismic shift in the national political landscape.

But the main opposition New Politics Alliance for Democracy (NPAD) was hoping that dissatisfaction with the Saenuri Party would translate into victories in a number of key mayoral and gubernatorial races.

Its strategy was to place the ferry tragedy front and centre of its campaign.

"If people don't act, the ship called the Republic of Korea will sink just like the Sewol did," NPAD leader Kim Han-Gil said on the eve of voting.

Opinion polls suggested the tactic could pay off.

In the most high-profile contest of the election -- the race for Seoul mayor -- the popular incumbent Park Won-Soon saw his lead over Saenuri rival Chung Mong-Joon widen into double figures following the Sewol disaster.

President Park's predecessor, Lee Myung-Bak, was a former Seoul mayor and there is speculation that whoever wins Wednesday's vote in the capital will use the position to set up a presidential run in 2017.

South Korea's second largest city Busan has always returned a conservative mayor, but this time around the Saenuri candidate, Suh Byung-Soo, was locked in a neck-and-neck race with an opposition-backed independent.

"Citizens of Busan, you made Park the president. Please help wipe away the tears she shed in the wake of the Sewol sinking," Suh said in a final appeal to voters.

His plea referenced a televised address Park made to the nation last month, in which she tearfully accepted responsibility for the mishandling of the disaster.

Several leading newspapers criticised both parties for pushing the idea of the election as a vote for or against Park.

"The election should be about evaluating the past performance of local governments and electing new local leaders," the Dong-A Ilbo said in an editorial.

"It is entirely up to voters to define what these elections should be about, regardless of what politicians say," it added.

Polling stations were set to close at 6:00pm (0900 GMT) and firm results expected later in the evening.