Volkswacht Bodensee - S. Korea's Yoon given until Saturday to appear for questioning: Yonhap

Yoon, suspended from office by parliament on Saturday, is being investigated over alleged insurrection by both South Korean prosecutors and a joint team of police, defence ministry and anti-corruption investigators.

The president and some of his inner circle face possible life imprisonment, or even the death penalty, if found guilty. He remains under a travel ban.

On Tuesday, prosecutors warned Yoon to appear for questioning over the martial law bid by Saturday or face a potential arrest warrant, Yonhap said citing the prosecution.

The joint investigation unit also asked Monday that the suspended president appear to answer questions on Wednesday but were rebuffed by his office, the news agency said.

South Korea's Constitutional Court on Monday began proceedings against Yoon and has around six months to determine whether to uphold his impeachment.

A spokeswoman for the court said judges had pencilled in a preliminary hearing on December 27, which Yoon is not required to attend.

Yoon was removed by South Korea's parliament on Saturday over his short-lived attempt to suspend civilian rule, which plunged the country into its worst political turmoil in years.

Fresh elections must be held within two months if his removal is upheld by the Constitutional Court. Prime Minister Han Duck-soo is serving as interim leader in Yoon's stead.

Vast protests against the ousted leader, with smaller rallies supporting him, have rocked the South Korean capital since his short-lived December 3 martial law decree.

Demonstrators in both camps have vowed to keep up the pressure as the Constitutional Court considers Yoon's fate.

On Monday night, hundreds of South Koreans held a vigil in central Seoul calling for his formal removal.

"I came here again, hoping that we will never have a president like this again," Kim Chan-suk, 67, told AFP at the rally.

"I am coming out every day to continue the fight until the Constitutional Court makes its ruling," protester Han Myung-hak, 52, told AFP.

J.Marty--VB