South Korea's acting president, Han Duck-soo, sought to reassure allies and stabilize financial markets on Sunday following the impeachment and suspension of President Yoon Suk Yeol over his attempt to impose martial law.

Han spoke with U.S. President Joe Biden by phone, the White House and Han's office said.

"South Korea will carry out its foreign and security policies without disruption and strive to ensure the South Korea-U.S. alliance is maintained and developed steadfastly," Han said, according to a statement from his office.

In a further attempt to stabilize the Asian nation's leadership, the main opposition party announced it would not seek to impeach Han for his involvement in Yoon's Dec. 3 martial law decision.

"Given that the prime minister has already been confirmed as acting president and considering that excessive impeachments could lead to confusion in national governance, we have decided not to proceed with impeachment procedures," Democratic Party leader Lee Jae-myung told reporters.

Prosecutors said Yoon did not appear Sunday morning in response to a summons for questioning in a criminal investigation into his martial law decision and they promised to issue another order, Yonhap news agency reported.

Yoon and a number of senior officials face potential charges of insurrection, abuse of authority and obstructing people from exercising their rights.

The prosecutors' office did not answer phone calls seeking comment.

Han, a longtime technocrat picked by the conservative Yoon as prime minister, was elevated to acting president in accordance with the constitution while Yoon's case moves to the Constitutional Court.

Demonstrators seeking Yoon's ouster braved the cold to throng the streets outside the National Assembly building where he was impeached. The crowd was about 200,000, according to police, Yonhap said.

Since Han's role is only acting president, "I hope he will exercise the minimum power to operate the country stably, rather than actively be involved in state affairs," said Jo Sung-woo, a 39-year-old Seoul resident.

About 8.5 kilometers (five miles) away, a much smaller number of Yoon supporters demonstrated in the central Seoul area.

"I really can't stand to see these illegal lawmakers who were elected by fraudulent elections making evil laws and now this huge opposition party is running wild on their own," said Yim Joung-sook, 55.

Pyongyang threat

Yoon's surprise martial law declaration and the ensuing political crisis spooked markets and South Korea's diplomatic partners, worried over the country's ability to deter nuclear-armed North Korea.

Biden told Han the ironclad U.S.-South Korea alliance remained unchanged and Washington would work with Seoul to further develop and strengthen the alliance as well as trilateral cooperation including neighbour Japan, Han's office said.

The White House said in a statement that the U.S. president "expressed his appreciation for the resiliency of democracy and the rule of law in the ROK," using the abbreviation for the country's formal name, the Republic of Korea.

Han convened his cabinet and National Security Council shortly after Saturday's impeachment vote and vowed to maintain military readiness to prevent any breach of national security.

He spoke by phone with the commander of U.S. Forces Korea, expressing concern about the possibility North Korea could attempt military provocations, such as launching ballistic missiles or cyber attacks, Yonhap said, citing Han's office.

South Korea’s partners wanted to see a credible and constitutional temporary leadership put in place as soon as possible, said Philip Turner, a former New Zealand ambassador to South Korea.

"They will be pleased to see Prime Minister Han take over as acting president," he said. "He is capable, experienced and well respected in foreign capitals."

But even with an acting president in place, international partners face months of uncertainty before a new president can be elected and a new government established, Turner added.

The Constitutional Court has up to six months to decide whether to remove or reinstate Yoon. If he is removed or resigns, a new elections will be held within 60 days.