S Korean investigators question arrested President Yoon

Impeached South Korean President Yoon Suk-yeol has been arrested and questioned for hours by investigators in relation to a criminal insurrection probe, ending a weeks-long stand-off with authorities.

Since MPs voted to impeach and strip him of his duties after his short-lived December 3 declaration of martial law, Yoon has been holed up at his hillside residence, guarded by a small army of presidential security staff who blocked a previous arrest attempt earlier this month.

On Wednesday he turned himself in for questioning at the corruption investigation offices after more than 3000 police officers seeking to arrest him marched on his residence before dawn.

"I decided to respond to the CIO's investigation - despite it being an illegal investigation - to prevent unsavoury bloodshed," Yoon said in a statement, referring to the Corruption Investigation Office for High-ranking Officials (CIO) that is heading the criminal inquiry.

A prosecutor accompanied Yoon in his car from his home in the upscale district known as Seoul's Beverly Hills to the austere CIO offices, where he slipped in through a back entrance, avoiding the media.

As Yoon was being interrogated, an unidentified man in his 60s set himself on fire nearby, fire services said.

The man was severely burned and was unconscious.

Authorities have 48 hours to question Yoon, after which they must seek a warrant to detain him for up to 20 days or release him.

However, Yoon is refusing to talk and has not agreed to have interviews with investigators recorded on video, a CIO official said.

The CIO said it had no information on why Yoon was refusing to talk.

The presidential motorcade was seen leaving the CIO offices late on Wednesday evening, with Yoon expected to be held at Seoul Detention Centre where other high-profile figures including former president Park Geun-hye and Samsung Electronics chairman Jay Y Lee have also spent time.

Yoon's lawyers have said the arrest warrant is illegal because it was issued by a court in the wrong jurisdiction and the team set up to investigate him had no legal mandate to do so.

A warrant to search Yoon at his residence, a copy of which was seen by Reuters, referred to him as "ringleader of insurrection".

MPs voted to impeach Yoon on December 14.

Separate to the criminal investigation, the Constitutional Court is deliberating whether to uphold Yoon's impeachment by parliament and permanently remove him from office or restore his presidential powers.

Wednesday's arrest attempt had hundreds of thousands of people glued to TV live feeds showing busloads of police arriving near the presidential residence, pushing past Yoon supporters and then walking towards the gates of the compound carrying ladders and wire cutters.

As local broadcasters reported that Yoon's detention was imminent, some minor scuffles broke out between pro-Yoon protesters and police near the residence.

Throngs of protesters, who also gathered outside the CIO offices, waved South Korean and US flags and held posters bearing "Stop the Steal" slogans referring to Yoon's unsubstantiated claims of election fraud - one of the reasons he gave to justify his martial law declaration.

In a hand-written letter posted to his Facebook page, Yoon said he was often called foolish because he did not compromise.

He said there was too much evidence of election fraud for it to be dismissed as a conspiracy theory, even while acknowledging "there is insufficient evidence to punish a specific person for it".

The National Election Commission - one target of the troops deployed by Yoon after he declared martial law - has denied allegations of fraud, saying the election system is transparent and above board with plenty of oversight.

Despite polls showing a majority of South Koreans disapprove of Yoon's martial law declaration and support his impeachment, the political stand-off has energised his supporters and his People Power Party (PPP) has apparently enjoyed a revival in recent weeks.

Support for the PPP stood at 40.8 per cent in the latest Realmeter poll released on Monday while the main opposition Democratic Party stood at 42.2 per cent, narrowing from a gap of 10.8 per cent last week.