Yoon, suspended from office by parliament since Saturday, faces concurrent probes over alleged insurrection by both South Korean prosecutors and a joint team of police, defence ministry and anti-corruption investigators.
South Korea’s Constitutional Court began proceedings against Yoon on Monday and has around six months to determine whether to uphold the impeachment.
Yoon and some of his inner circle face possible life imprisonment, or even the death penalty, if found guilty and he remains under a travel ban.
Prosecutors issued Yoon with a second summons on Monday for questioning over insurrection and abuse of power allegations after he “refused to comply” with a similar request a day earlier.