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 former Prime Minister Han Duck-soo says he will run in next month's presidential election and would seek to lessen the powers of the office and ease strife-ridden domestic politics if he wins.

South Korea's third acting president in five months Lee Ju-ho said on Friday he will ensure stable government ahead of a June 3 snap presidential election, following the quick-fire resignations of the prime minister and finance minister.