South Korea's opposition party urges Yoon to resign or risk impeachment

South Korea's main opposition party has said it would initiate impeachment proceedings against President Yoon Suk-yeol unless he immediately resigned after his short-lived declaration of martial law overnight.

If Yoon "does not resign immediately, the Democratic Party will immediately initiate impeachment proceedings in line with the will of the people," it said in a statement on Wednesday.

"His martial law declaration was originally invalid and a grave violation of the constitution. It was a grave act of rebellion and provided perfect grounds for his impeachment."

Impeaching Yoon would require support from two-thirds of the parliament (200 of its 300 members). The Democratic Party and other small opposition parties together have 192 seats. But when the parliament rejected Yoon's martial law declaration in a 190-0 vote, about ten lawmakers from Yoon's ruling People Power Party cast ballots supporting the rejection, according to National Assembly officials.

If Yoon is impeached, he'll stripped of his constitutional powers until the Constitutional Court can rule on his fate. Prime Minister Han Duck-soo, the No. 2 position in the South Korean government, would take over his presidential responsibilities.

For their part, Senior aides working for Yoon offered to resign en masse, the Yonhap News Agency reported.

"Yoon's senior aides offer to resign en masse over martial law declaration," Yonhap said, without giving further details.

Deep turmoil

Yoon's martial law declaration, the first of its kind in more than 40 years, harkened to South Korea’s past military-backed governments when authorities occasionally proclaimed martial law and other decrees that allowed them to station combat soldiers, tanks and armoured vehicles on streets or at public places like schools to prevent anti-government demonstrations.

His shock bid plunged the country into the deepest turmoil in its modern democratic history and caught its close allies around the world off guard.

Yoon backed down after lawmakers voted to oppose the declaration, which he made late on Tuesday night, citing the threat of North Korea and "anti-state forces".

Protesters outside the National Assembly parliament shouted and clapped. "We won!" they chanted.

South Korea's largest union coalition, the Korean Confederation of Trade Unions, said thousands of its members would strike until Yoon resigned and hold a rally later in the day in downtown Seoul.

Several other protests are expected, including near the National Assembly, where thousands of people gathered late on Tuesday to call to block Yoon's order and then demand his arrest and resignation.