South Korea's ferry disaster has cast a heavy shadow over upcoming local elections, especially the key race for Seoul mayor -- a high-profile post seen as a possible springboard for the presidency.

With the nation still reeling from the April 16 tragedy that claimed around 300 lives, mostly schoolchildren, next week's polls for city councils, mayors and provincial governors offer a chance to measure the extent of the political fallout.

Surrounded by allegations of negligence, greed and incompetence, the sinking of the 6,825-tonne Sewol ferry triggered intense criticism of the government and President Park Geun-Hye whose ruling conservative Saenuri Party fears a backlash in the June 4 vote.

Particular attention will be paid to the mayoral race in Seoul where the two main candidates for one of the most powerful jobs in the country present a timely study in contrasts.

The anti-establishment mood that has swelled in the wake of the Sewol disaster would seem to favour the incumbent, Park Won-Soon, who is standing for a second term.

A former civil rights activist, Park was a something of a political non-entity when he made history by winning the 2011 mayoral election as an independent -- although he has since joined the main opposition grouping.

Park was a generally popular mayor, engaging with voters on social networks and promoting community-driven governance while eschewing the marquee construction projects of some of his predecessors.

His main challenger, tycoon-turned-politician Chung Mong-Joon of the Saenuri Party, is a seven-term legislator and scion of the family that founded and controls the giant Hyundai conglomerate.

The ferry tragedy has loomed large over the election campaign, with both candidates hammering home the need to improve public safety.

"The disaster was a huge wake-up call and revealed the darker shadows of our society's rapid development and growth-oriented impatience," Park told AFP in an interview, during which he repeatedly underlined his civil activist past and image as a political outsider.

Safety at heart of campaign battle

Park believes South Korea is in a historic period of transition, with people seeking to move away from a hyper-competitive society to one more focused on quality of life.

"Ordinary citizens want change and the Sewol accident has shown it's time to review our successes and mistakes and seek a new direction," he said.

"Korea will never be the same," he added.

Park described his rival as a "professional politician" stuck in the ways of the past and "aloofly" unaware of the shift in national sentiment.

"Obviously I'm a politician now, but people still see me as a civil rights leader," he said.

The idea that Park's candidacy is more in tune with current sentiment has been borne out by opinion polls showing his lead over Chung widening into double digits since the ferry disaster.

Chung was not helped by having to apologise after his son described public criticism of President Park over the ferry disaster as "uncivilised".

In a briefing for foreign journalists this week, Chung prefaced his remarks by requesting a moment's silence for the ferry victims and argued that he was the candidate who would "keep Seoul safe".

He also cited a Seoul subway collision earlier this month as evidence of Park's "indifference" to safety issues.

There were no casualties but hundreds of passengers sustained minor injuries and the incident was lent extra weight by coming so soon after the Sewol sinking.

In a reflection of how safety has become the dominant campaign issue, Park cited the collision as the "most regrettable incident" of his first term.

It remains to be seen just how great the impact of the ferry tragedy will be on the outcome of next week's election in Seoul and elsewhere, and whether or not any public shift will be long-lasting.

Both Park and Chung are believed to have presidential aspirations, and the latter has already taken runs at the Blue House in 2002 and 2012, only to drop out midway through both races.

Park Geun-Hye's predecessor as president, Lee Myung-Bak, was a former Seoul mayor and there is widespread speculation that whoever wins Wednesday's vote would use the position to set up a presidential run in 2017.

For the moment, however, both candidates insist their only ambition is to run the capital, not the country.