The leader of South Korea's main opposition party has been taken to hospital after his health deteriorated on the 19th day of a hunger strike.

The Democratic Party said on Monday that Lee Jae-myung was transported to hospital earlier in the day.

Lee began fasting in late August to protest the policies of President Yoon Suk-yeol, including the response to Japan's release of treated and diluted water from the Fukushima Daiichi nuclear power plant.

South Korean media report that Lee was fasting in a tent in front of the National Assembly, but rested in his office at night.

The ruling People Power Party ridiculed Lee for commuting to his hunger strike, and said his action lacked cause or reason.

After Lee was rushed to hospital, the ruling party said he had dragged the National Assembly into political strife.

Lee began the hunger strike amid corruption allegations against him.

Prosecutors interviewed him last week over his alleged involvement in a company's illegal cash remittance to North Korea.

And on Monday, prosecutors requested an arrest warrant for him on suspicion of bribery and breach of trust over a land development project when he was the mayor of Seongnam City.