South Korea has sworn in its third acting president in less than six months, as his predecessor declared his candidacy in a snap election to replace impeached ex-leader Yoon Suk-yeol.

South Korea's top court cast doubt on Thursday on frontrunner Lee Jae-myung's eligibility to run for the presidency, while the resignations of the prime minister and finance minister shook the interim government in place since December's martial law.

Prime Minister Han Duck-soo has been mulling whether to contest the election, while the main opposition candidate and front-runner suffered a setback in court.