A fish merchant is critically injured after setting himself ablaze on Friday during a protest over compensation for South Korea's worst oil spill, a medical official said.

Ji Chang-Hwan, 56, drank a bottle of herbicide and doused himself with paint thinner before setting himself on fire, witnesses said.

"He is in critical condition, especially because he drank herbicide," an official at Taean Medical Centre said, adding that the man was being transferred to a large hospital in nearby Cheonan city.

Last week another man committed suicide by drinking poison after his oyster farm was destroyed by the oil spill.

Ji was among 5,000 people taking part in a rally in Taean county, demanding special laws aimed at compensating them fully for damage from the oil spill off the west coast last month.

A barge drifting in stormy weather smashed into the 147,000-ton Hong Kong-registered tanker Hebei Spirit on December 7, causing the tanker to spill some 10,900 tonnes of crude oil.

The spill destroyed scores of sea farms and polluted miles of beaches along the coast.

Tens of thousands of police, troops and volunteers have staged a huge clean-up of the shoreline but environmentalists say the damage could last for years.

Police plan to announce next week the results of their investigation into the accident.