The Constitutional Court of South Korea began formal deliberations on Tuesday to decide whether to unseat President Yoon Suk Yeol. The impeached leader stayed away, holed up in his fortified presidential compound where he is bracing himself for what his aides called a “siege attack” from up to 1,000 criminal investigators planning to detain him on insurrection charges.

Mr. Yoon has been suspended from office since he was impeached by the National Assembly on Dec. 14 for his short-lived imposition of martial law 11 days earlier. But he has refused to step down. Instead, he vowed to “fight to the end” to regain power through a trial at the Constitutional Court, and has resisted demands he submit to questioning by officials conducting a separate investigation of insurrection charges.

The ongoing efforts to bring Mr. Yoon to account for declaring military rule and his refusal to cooperate with all investigations so far has left South Korea in political limbo, sowing doubts over the resilience of its decades-old democracy.

The Constitutional Court has the sole power to decide whether the parliamentary impeachment was legitimate and whether Mr. Yoon should be formally removed or reinstated. Small but loud rival groups of citizens engaged in shouting matches across the narrow street in front of the court on Tuesday as the first hearing opened. The perimeter wall of the court was lined with flowers that Mr. Yoon’s supporters had sent.

But Mr. Yoon did not show up: His lawyers said he feared that the insurrection investigators would nab him if he left his presidential compound. The court adjourned after four minutes on Tuesday when it found Mr. Yoon absent. It said it would resume its deliberations on Thursday, when it can proceed with or without him.