South Korea's main opposition party said Sunday it will try again to impeach President Yoon Suk Yeol after his declaration of martial law.

Meanwhile police arrested the defence minister in charge of the martial law operation, and the interior minister resigned. Both they and Yoon are being investigated for alleged insurrection.

Yoon averted impeachment late Saturday as huge crowds braved freezing temperatures in another night of protests outside parliament to demand the president's ouster.

Opposition parties proposed the impeachment motion, which needed 200 votes in the 300-member parliament to pass, but a near-total boycott by Yoon's People Power Party (PPP) doomed it to failure.

Lee Jae-myung, leader of the main opposition Democratic Party (DP), said Sunday that they will try again on 14 December.

"Yoon, the principal culprit behind the insurrection and military coup that destroyed South Korea's constitutional order, must either resign immediately or be impeached without delay," Lee told reporters. 

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

'Soft coup'

In exchange for blocking his removal from office, Yoon's People Power Party (PPP) said that it had "effectively obtained (Yoon's) promise to step down".

"Even before the president steps down, he will not interfere in state affairs, including foreign affairs," PPP leader Han Dong-hoon said Sunday after a meeting with Prime Minister Han Duck-soo.

This will "minimize the confusion to South Korea and its people, stably resolve the political situation and recover liberal democracy," Han told reporters.

But Lee and the National Assembly speaker Woo Won-shik, both from the opposition Democratic Party (DP), on Sunday called the arrangement illegal.

"For the prime minister and the ruling party to jointly exercise presidential authority, which no one has granted them, without participating in constitutional processes to address unconstitutional martial law, is a clear violation of the Constitution," Woo said.

"The power of the president is not the personal property of President Yoon Suk Yeol," said Lee. "Isn’t this another coup that destroys the constitutional order?"

Kim Hae-won, a constitutional law professor at Pusan National University Law School, called it a an "unconstitutional soft coup."

"In reality, a political party is merely a private political entity, and handing over the president's functions to an entity that is neither a constitutional institution nor a state body seems like an action that disrupts the state's rights," Kim told AFP.