SEOUL - South Korean prosecutors Sunday booked President Yoon Suk-yeol as a suspect on charges of treason, Yonhap news agency reported.

Park Se-hyun, who leads a special investigation headquarters into Yoon's short-lived martial law declaration, said the prosecution is investigating charges of treason and abuse of power against the president.

"Numerous complaints have been submitted, and an investigation is underway in accordance with the process," Park said.

While the president mostly has immunity from prosecution while in office, that does not extend to allegations of rebellion or treason.

Earlier in the day, prosecutors arrested former Defense Minister Kim Yong-hyun, who allegedly recommended Tuesday's brief martial law imposition to Yoon, making him the first figure detained over the case.

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The main opposition Democratic Party of Korea Sunday called for the immediate arrest and investigation of Yoon over his martial law decree.

Meanwhile, National Assembly Speaker Woo Won-shik said the prime minister and the ruling party's joint exercise of presidential power is "unconstitutional."

Woo made the remarks during a press conference at the National Assembly, just hours after Prime Minister Han Duck-soo and ruling People Power Party leader Han Dong-hoon issued a joint statement, affirming President Yoon would not be involved in the country's internal and foreign affairs before he steps down.

Woo said that presenting the joint statement is an act of misleading the public as it makes an unconstitutional act look legitimate, calling it "arrogant."

"The transfer of presidential power comes from the people, and such a process should abide by the Constitution and the principle of popular sovereignty," Woo said, adding "Impeachment is the only legal procedure to suspend the president from carrying out his duties."

The parliament speaker also proposed talks between the ruling party and the main opposition to discuss the immediate suspension of Yoon's duties and resolve the political turmoil following Yoon's short-lived declaration of martial law Tuesday.

In a further development Sunday, President Yoon accepted the resignation of Minister of the Interior and Safety Lee Sang-min.

"I believe stepping down is necessary to prevent further disruptions and confusion in governance," Lee said in a statement.

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Lee's resignation followed an impeachment motion filed by the Democratic Party on Saturday, which accused him of complicity in the declaration of martial law and mobilizing police to block lawmakers from entering the National Assembly.

The Democratic Party also vowed to keep pursuing a motion to impeach Yoon after the first one was scrapped on lack of quorum earlier in the day.

"Yoon ... must either resign immediately or be impeached without delay," Lee Jae-myung, leader of the Democratic Party, said in a televised speech.

"On Dec 14, our Democratic Party will impeach Yoon in the name of the people," he added.

The first impeachment motion against Yoon was scrapped as most of the 108 governing party lawmakers boycotted it.

The Democratic Party and five other minor parties submitted the impeachment motion over the president's martial law declaration Wednesday.