Beleaguered South Korean president Yoon Suk Yeol said the country had been facing an "existential crisis" when he declared martial law in December last year.

Yoon remained defiant as as he faced Seoul's eight-judge strong Constitutional Court reviewing whether to uphold his impeachment over the short-lived martial law declaration.

If the court upholds the parliament's decision to impeach Yoon, he will be removed from office less than three years into a five-year term.

According to Yoon, "external forces, including North Korea, along with anti-state elements" within South Korean society were "working together to seriously threaten our national security and sovereignty".

"This was never a decision made for my personal benefit as Yoon Suk Yeol," he told the court.

Yoon's defense cites Trump example

Kicking off the hearing, Yoon's defense team argued that the ousted president cannot be punished for "exercising his core constitutional powers".

They cited a 2024 US Supreme Court ruling, Donald Trump v. the United States where the court ruled that former US presidents enjoy absolute criminal immunity for "core constitutional" powers. Trump had escalated the matter of immunity to the top court when lower courts had refused to grant him immunity for his role in the 2020 Capitol Riots.

That ruling "should be considered in the context of impeachment proceedings", Yoon's lawyer Lee Dong-chan said.

In response, prosecutor Lee Gum-gyu said he felt betrayed as his son, an active duty soldier, would have been forced to participate in Yoon's martial law.

What happens next as the impeachment trial wraps up

A verdict is widely expected in mid-March. If impeached, South Korea will have to elect a new president within the next 60 days.

Yoon is also facing a parallel criminal trial over insurrection charges related to the martial law declaration.

He is the first sitting South Korean president to stand trial in a criminal case. The case is expected to drag on well past his impeachment.

Yoon has argued that he did not mean to impose full military rule. He said he had the right to impose martial law but only meant to sound an alarm over the opposition's abuse of a parliamentary majority.

He has further argued that there is no point in debating accusations that he had ordered the military to enter the parliament and remove lawmakers gathering to rescind the martial law decree, since "nothing actually happened" and nobody was harmed.

Meanwhile, the South Korean parliament argued that Yoon's judgment on what called for martial law was compromised and he could repeat his actions if reinstated.

Edited by: Louis Oelofse and Zac Crellin