Many of the tens of thousands of protesters outside South Korea's parliament were overcome with emotion on Saturday, as a presidential impeachment motion against President Yoon Suk Yeol looked set to fail.

As the plenary session got under way in South Korea’s national assembly, some members of the ruling People Power Party (PPP) walked out of the chamber after voting on the special counsel appointment in the first lady’s case.

Hours later, some ruling PPP lawmakers returned to cast their votes, telling reporters they voted against the impeachment and that they returned to vote because it was "important".

Nearly 150,000 people were estimated by police to have filled the streets around the National Assembly on Saturday, demanding that lawmakers impeach Yoon for his sudden imposition of martial law four days earlier, which plunged South Korea into turmoil.

For hours, people poured into the area around parliament where on Tuesday night soldiers were helicoptered in as part of Yoon's short-lived bid to subvert civilian rule.

Ahead of the vote on the opposition-led impeachment motion, giant screens to show parliament livefeeds were set up across the eight-lane road by the National Assembly, which had been closed off to serve as a rally site.

In the event that the impeachment vote is passed, Yoon would be immediately suspended and replaced as president by the prime minister, Han Duck-soo. It would then be left to the Constitutional court to decide whether to uphold the national assembly’s vote. (Agencies)