South Korea's Yoon, facing unprecedented arrest over martial law, vows to 'fight until end'

South Korea's impeached President Yoon Suk Yeol has sent a letter rallying his supporters saying he would "fight until the end" as he faces an attempt by authorities to arrest him over his short-lived Dec. 3 martial law, a lawyer said on Thursday.

"I am watching on YouTube live all the hard work you are doing," Yoon wrote in the letter late on Wednesday to the estimated hundreds of supporters who have gathered near his official residence protesting his investigation.

"I will fight until the end to protect this country together with you," he said in the letter, a photo capture of which was sent to Reuters by Seok Dong-hyeon, a lawyer advising Yoon.

The opposition Democratic Party, which has majority control of parliament and led the impeachment of Yoon on Dec. 14, said the letter proved Yoon was delusional and remains committed to complete his "insurrection"

"As if trying to stage insurrection wasn't enough, he is now inciting his supporters to an extreme clash," party spokesman Jo Seoung-lae said in a statement.

A court on Tuesday approved a warrant for Yoon's arrest, which potentially would make him the first sitting president to be detained as part of investigations over allegations he masterminded insurrection by trying to impose martial law.

Insurrection is one of the few criminal charges from which a South Korean president does not have immunity.

The Corruption Investigation Office for High-ranking Officials (CIO), which is leading a joint team of investigators that include the police and prosecutors, has until Jan. 6 to execute the arrest warrant.

It was unclear when and how it will make the arrest and whether the presidential security service, which has blocked access by investigators with a search warrant to Yoon's office and official residence, would try to stop the arrest attempt.

Separately, Yoon's trial on impeachment is being heard at the Constitutional Court. The court will hold the second hearing on Friday. Yoon has been suspended from presidential duties and Finance Minister Choi Sang-mok has taken over as acting president until the outcome of the trial.

If the court upholds the impeachment and Yoon is removed from office, a new presidential election will be held within 60 days.

Yoon Kab-keun, a lawyer for the impeached president, said the arrest warrant was illegal and invalid because the CIO did not have the authority under South Korean law to request a warrant.

The warrant for Yoon's arrest and also a search of his office and residence was issued after the conservative career prosecutor defied repeated summons by investigators to appear for questioning in the criminal investigation separate from the Constitutional Court trial.

A former defence minister who officials said recommended Yoon declare martial law has been indicted on charges of insurrection and will go on trial on Jan. 16. Some of the top military officers commanding the defence of the capital, Seoul, have also been indicted for their alleged involvement.