South Korea's main opposition party on Wednesday 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.

A coalition of lawmakers from opposition parties said they planned to propose a bill to impeach Yoon on Wednesday which should be voted within 72 hours.

“The parliament should focus on immediately suspending the president’s business to pass an impeachment bill soonest,” Hwang Un-ha, one of the MPs in the coalition, told reporters.

Yoon's chief of staff and senior secretaries offered to resign en masse, a presidential official said.

Yoon told the nation in a TV address that martial law was needed to defend the country from nuclear-armed North Korea and pro-North anti-state forces, and protect its free constitutional order, although he cited no specific threats.

Chaotic scenes ensued as helmeted troops climbed into the parliament building through smashed windows and military helicopters hovered overhead.

Parliamentary aides sprayed fire extinguishers to push the soldiers back, and protesters scuffled with police outside.

The military said activities by parliament and political parties would be banned, and that media and publishers would be under the control of the martial law command.

But within hours of the declaration, South Korea's parliament, with 190 of its 300 members present, unanimously passed a motion requiring martial law be lifted, including all 18 members present from Yoon's party.

The president then rescinded the declaration.

Protesters outside the National Assembly shouted and clapped. “We won!” they chanted, and one demonstrator banged on a drum.

More protests are expected on Wednesday with South Korea's largest union coalition, the Korean Confederation of Trade Unions, planning to hold a rally in Seoul and vowing to strike until Yoon resigns. (Agencies)