South Korea's ousted President Yoon Suk-yeol faced his final impeachment hearing on Tuesday before judges decide whether to formally remove him from office over his disastrous martial law declaration.

Yoon's short-lived suspension of civilian rule plunged democratic South Korea into political turmoil, and he was removed from office by parliament in December.

The 64-year-old has been behind bars since he was arrested last month on charges of insurrection, for which he could be sentenced to life in prison or even face the death penalty.

His trial began last week.

After weeks of fraught impeachment hearings at the Constitutional Court in Seoul, Tuesday's court session was his last before the eight judges go behind closed doors to decide his fate.

Proceedings began at 2pm but Yoon was not present, reporters said.

A number of lawmakers from his ruling People Power Party were in attendance, however.

In opening remarks, Yoon's defence team cited a 2024 US Supreme Court ruling, Donald Trump v. the United States, arguing the ousted president cannot be punished for "exercising his core constitutional powers".

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

Yoon is expected to deliver a closing argument in his defence, with representatives of parliament given time to present the case for his removal.

Outside the court, pro-Yoon protesters chanted "Drop impeachment!"

Others held signs saying "Stop the Steal", echoing US President Trump's false claims of voter fraud when he lost the 2020 election to Joe Biden.

A verdict is widely expected in mid-March.

Previously impeached presidents Park Geun-hye and Roh Moo-hyun had to wait 11 and 14 days, respectively, to learn their fates.

If Yoon is removed from office, the country must hold a fresh presidential election within 60 days. (AFP)