South Korea's parliament impeached acting President Han Duck-soo on Friday, less than two weeks after suspending President Yoon Suk Yeol's powers over his short-lived declaration of martial law, plunging the country deeper into political chaos.

The impeachment of prime minister Han, the acting president since Yoon was impeached on Dec. 14 for declaring martial law on Dec. 3, has pushed South Korea's once-vibrant democratic success story into uncharted territory.

Finance Minister Choi Sang-mok, who assumed the position of acting president while the cases of Yoon and Han are considered by the Constitutional Court, convened the National Security Council, spoke with key officials including military leaders and vowed to do everything in his power to stabilise state affairs.

The unexpected imposition of martial law and the ensuing political upheaval sent shockwaves through Asia's fourth-largest economy, and drew concerns from allies in the United States and Europe who had seen Yoon as a key partner in efforts to counter China, Russia and North Korea.

Han's sudden ouster adds to the uncertainty, and Choi may also face removal if he too clashes with the opposition-led parliament.

"The government must do its best to ensure that the people do not become anxious, or the security of the country and people's daily lives are not shaken," Choi said, according to a statement from his office.

Earlier, Choi had pleaded unsuccessfully with parliament to withdraw the plan to impeach Han, saying it would do serious damage to the economy.

The Korean won was down 0.5% at 1,477.0 per dollar as of 1100 GMT, after hitting a more than 15-year low of 1,486.7 ahead of the vote.

"In terms of financial markets, (Choi) taking charge can only be bad news, as it only goes to show that political turmoil is ongoing," said Huh Jae-hwan, an analyst at Eugene Investment & Securities.

The country could plunge into economic troubles comparable to its devastating financial crisis of the late 1990s, said Shin Yul, a political science professor at Myongji University.

CONSTITUTIONAL FIGHT

The impeachment of Han came after he declined to immediately appoint three justices to fill vacancies at the Constitutional Court, saying it would exceed his acting role.

Han said he accepted the outcome. "...In order to avoid further chaos and uncertainty, I will suspend my duties in accordance with relevant laws," he said.

He added he would await the decision of the Constitutional Court to review the impeachment motion.

The motion led by opposition parties passed with 192 votes in favour and no opposed after the ruling People Power Party (PPP) boycotted the vote. PPP members surrounded the speaker's podium, chanting that the vote was invalid and parliament had engaged in "tyranny".

Ahead of the parliamentary session, opposition leader Lee Jae-myung of the Democratic Party, which has majority control of parliament, accused Han of "acting for insurrection".

Until just before voting began, it was unclear how many votes were needed to impeach Han. The threshold for a prime minister is a simple majority, while a two-thirds majority is needed for a president.

Speaker Woo Won-shik declared a simple majority would constitute parliamentary approval, but the People Power Party said it had filed a constitutional court petition declaring that such a threshold was too low to remove an acting president.

SWIFT TRIAL

Also on Friday, the Constitutional Court held its first hearing to review whether to reinstate Yoon or remove him permanently from office. It has 180 days to reach a decision.

At a preparatory hearing, Justice Cheong Hyung-sik denied a request by Yoon's lawyers for a postponement in proceedings to better prepare and said the court would move swiftly.

The next hearing is due on Jan. 3.

A lawyer representing Yoon later told reporters the impeached president planned to appear in person in future.

He was not required to attend Friday's hearing. He has previously ignored court requests to submit documents as well as summons in a separate criminal case over his martial law declaration.

If he is ousted, a new presidential election would be held within 60 days.

There has been overwhelming public support for Yoon's removal, opinion polls showed after his martial law attempt.

WORST POLITICAL CRISIS IN DECADES

Yoon shocked the country and the world with a late-night announcement on Dec. 3 that he was imposing martial law to overcome political deadlock and root out "anti-state forces".

Within hours, however, 190 lawmakers had defied the cordons of troops and police and voted against Yoon's order. About six hours after his initial decree, Yoon rescinded it.

Events since the declaration have sparked South Korea's gravest political crisis since 1987, when protests forced the ruling party of former military generals to accept a direct, popular vote to elect the president.

On Friday, prosecutors indicted former Defence Minister Kim Yong-hyun in the first move to put on trial an official accused of insurrection, Yonhap news said.