South Korea's opposition leader urged a top court on Sunday to swiftly formalize the impeachment of President Yoon Suk Yeol and ease the "suffering of the people" following his short-lived martial law decree.

Lawmakers voted on Saturday to remove Yoon from office over his "insurrectionary" suspension of civilian rule, which lasted only hours but plunged South Korea into some of its worst political turmoil in years.

Yoon has been suspended while South Korea's Constitutional Court deliberates the impeachment, with Prime Minister Han Duck-soo serving as interim leader. The court has 180 days to rule on Yoon's future.

Opposition leader Lee Jae-myung on Sunday urged the judges to "swiftly" remove Yoon from office.

"This is the only way to minimise national turmoil and alleviate the suffering of the people," he said.

"To hold those responsible for this absurd situation accountable and to prevent its recurrence, it is essential to uncover the truth and demand accountability."

An investigation into Yoon's inner circle over last week's martial law declaration has also continued.

On Sunday, police arrested both the current and former heads of the Defense Intelligence Command in connection with the insurrection allegations, Yonhap news agency reported.

Prosecutors said they were also seeking an arrest warrant for the head of the Army Special Warfare Command, Kwak Jong-keun, according to Yonhap.

Kwak is accused of sending special forces troops to parliament during the martial law bid, sparking a dramatic confrontation between soldiers and parliamentary staff.

Investigators also said they had summoned Yoon for questioning over the insurrection allegations.

"We instructed him to appear for questioning at 10 am (GMT 0100), but he refused to comply," the prosecution said in a press release.

"We will issue a second summons," they added without providing further details.

On Saturday, police also arrested Yeo In-hyung, head of the Defense Counterintelligence Command, on charges including insurrection.

'Resiliency of democracy'

The South Korean government has, meanwhile, sought to project an air of business as usual.

Acting President Han on Sunday held a call with U.S. President Joe Biden, who underscored the strength of bilateral ties.

The United States is a key treaty ally of Seoul and has around 28,000 troops stationed in South Korea.

The White House said in a readout that Biden had expressed "confidence that the Alliance will remain the linchpin for peace and prosperity in the Indo-Pacific region during Acting President Han's tenure."

"President Biden expressed his appreciation for the resiliency of democracy and the rule of law in the Republic of Korea," the White House said.

Han has also ordered the military to "enhance vigilance" against North Korea, with which the South technically remains at war.

North Korea has yet to publicly comment on Yoon's impeachment.

Mass protests both for and against Yoon have rocked the South Korean capital since the December 3 martial law decree.

Demonstrators in both camps have vowed to maintain their pressure campaigns as the Constitutional Court mulls Yoon's fate.

"I will certainly protest at the court to demand it reject the impeachment," Cho Hee-sun, a Yoon supporter, told AFP at a rally on Saturday before the parliamentary vote.

Seoul police estimated that at least 200,000 people gathered outside parliament in support of removing the president.