As president of South Korea, Yoon Suk Yeol lived in a luxurious hilltop mansion, threw parties and had a small army of personal guards. These days, he is alone in a 107-square-foot jail cell, eating simple food like noodles and kimchi soup, and sleeping on the floor.

       This will be his new reality for a while yet, after he was formally arrested on insurrection charges early Sunday as part of an investigation into his ill-fated declaration of martial law last month.

       Mr. Yoon, 64, has been in the Seoul Detention Center, a government-run jail south of Seoul, since Wednesday, when he became the first sitting president in South Korean history to be detained in a criminal investigation. When a district court in Seoul issued the warrant to arrest him, he went from being a temporary detainee to a criminal suspect facing an indictment and trial.

       That change in status means Mr. Yoon is unlikely to leave jail any time soon. Within the next 18 days, criminal investigators and prosecutors are expected to indict him on charges of leading an insurrection during his short-lived martial law last month. If he is convicted, he will face life imprisonment or ？the death penalty.

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       Mr. Yoon’s new circumstances symbolize his dramatic fall from grace: from a swaggering head of state to an impeached president to an inmate accused of committing one of the worst offenses in South Korea’s criminal code. He is the first South Korean to face insurrection charges since the former military dictator Chun Doo-hwan, who was convicted in the 1990s.

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       Mr. Yoon being transported from his presidential mansion on Wednesday.Credit...Jun Michael Park for The New York Times

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