"We've submitted an impeachment motion prepared urgently," representatives for six opposition parties including the main Democratic party said on Wednesday, adding they would discuss when to put it to a vote, but it could come as soon as Friday.
Earlier on Wednesday, Yoon faced calls to quit immediately or face impeachment after an attempt to bring in martial law triggered protests and political condemnation. The liberal opposition Democratic party, which holds a majority in the 300-seat parliament, said its MPs had decided to call on Yoon to stand down straight away or they would take steps to impeach him.
"President Yoon Suk Yeol's martial law declaration was a clear violation of the constitution. It didn't abide by any requirements to declare it," the Democratic party said in a statement. "His martial law declaration was originally invalid and a grave violation of the constitution. It was a grave act of rebellion and provides perfect grounds for his impeachment."
In a further development on Wednesday evening, defence minister Kim Yong-hyun offered his resignation, while simultaneously facing an impeachment motion from the Democratic party. If Yoon accepts Kim's resignation before parliament votes, the defence minister would no longer be subject to the impeachment process.
Yoon's shock bid to impose South Korea's first state of martial law in over four decades plunged the country into the deepest turmoil in its modern democratic history and caught its close allies around the world off guard.
The US - which stations nearly 30,000 troops in South Korea to protect it from the nuclear-armed North - voiced deep concern at the declaration, then relief that martial law was over.
The US indefinitely postponed meetings of the nuclear consultative group (NCG), a signature Yoon effort aimed at having South Korea play a greater role in allied planning for potential nuclear war on the peninsula.
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