"The normalization of the Republic of Korea is urgent," Lee told a news conference, as quoted by the news agency.
The official also said that the Democratic Party would actively cooperate with all political parties "for the stabilization of state affairs and to recover international trust," as quoted by Yonhap.
At the same time, the opposition party decided not to take impeachment steps against South Korean Prime Minister Han Deok-soo, who had assumed duties as acting president, Lee added.
On Saturday, the parliament supported the impeachment of the president after 204 lawmakers voted in favor of the top official's removal from office, while 85 opposed. The first parliamentary vote to impeach Yoon was held on December 7, but failed after the ruling party boycotted the vote.
Shortly after, Yoon lifted martial law and apologized to the nation. He was banned from leaving the country pending the results of an investigation, while former Defense Minister Kim Yong-hyun was arrested on suspicion of treason.