South Korea's impeached president fends off arrest attempt over martial-law decree

Yoon, who is being investigated for insurrection among other charges, has refused three summonses to appear for questioning. He views the legal scrutiny as illegitimate. On Tuesday, a Seoul court issued a detainment warrant that would bring the 64-year-old leader in front of investigators.

In recent days, Yoon and his legal-defense team had lambasted the warrant as “illegal and invalid," demanding the issuing judge be investigated.

The arrest warrant is valid through Monday. Before then, the special unit handling the probe—the Corruption Investigation Office for High-Ranking Officials—could attempt again to detain the conservative Yoon, who is at South Korea’s presidential residence in central Seoul. They could also ask the court to reissue the warrant after its expiration.

The political tumult lands at a sensitive time for Seoul, with a looming change in U.S. administrations, an increasingly bellicose North Korea and a plane crash in South Korea that killed 179 people. Yoon was stripped of his presidential powers after his Dec. 14 impeachment. Then last week, his immediate successor, Prime Minister Han Duck-soo, was also impeached.

The special investigators had threatened that presidential security personnel who helped Yoon defy arrest could be charged with dereliction of duty or obstruction. They view the execution of the arrest warrant as part of routine judicial procedure.

By law, the mission of the presidential security detail is to protect the country’s leader—and Yoon, impeached though not removed from office, remains the elected head of the country pending a Constitutional Court review. The court has until mid-June to do so, though it is expected to move more quickly than that.

Yoon’s security team appears to view the arrest as a threat to the safety of the president, said Kim Seon-taek, of Korea University in Seoul, who specializes in constitutional law. “This is why there is a conflict," he said.

The inability to arrest Yoon is likely to invite more political unrest. Dueling mass protests have unfolded across South Korea in recent weeks, backing or disavowing the impeachment. Yoon’s backers, who include influential far-right YouTubers, have called for an “army of people" to defend him.

Investigators have been blocked from carrying out search-and-seizure raids by Yoon’s Secret Service team. On Thursday, law enforcement forcibly removed about 30 pro-Yoon protesters who had broken through a police barrier and sought to block access to the entrance of the presidential residence.

In a letter sent to his backers the prior day, Yoon said he appreciated their efforts, which he watched via YouTube livestreams. Supporters, chanting “Let’s protect Yoon," have blocked alleyways and a police bus and physically attacked law enforcement.

South Korea is “now in danger" due to the activities of antistate groups, Yoon said in his letter. “With you, I will fight to the end to protect this country," it read.

Yoon’s legal woes run parallel with his impeachment trial, which formally started last week. The Constitutional Court plays a similar role in impeachment cases to the U.S. Senate.

After Yoon’s impeachment, Han, the prime minister, assumed the nation’s top job. He was impeached by the opposition-controlled National Assembly after refusing to appoint judges to the Constitutional Court, which had only six of nine seats filled.

The whirlwind of events has left Choi Sang-mok, the No. 3 in line, with four of the nation’s top positions: acting president, acting prime minister and his previous roles as deputy prime minister and head of the country’s economy and finance ministry.

On Tuesday, Choi appointed judges for two of the three vacant seats at the Constitutional Court, doing so to “end the political uncertainty and social conflict as soon as possible." That drew immediate backlash from the ruling conservatives; many top presidential aides who had served Yoon tendered their resignations.

Choi, as acting president, could direct the Secret Service team to step aside and allow Yoon to be arrested, said Korea University’s Kim. But Choi has so far not commented publicly on the matter.

If the Constitutional Court upholds his impeachment, Yoon, about halfway through a five-year term set to end in 2027, will be ousted from office and lose the presidential immunity that shields South Korean leaders from everything but a few charges, such as insurrection or treason.

A snap election would occur within 60 days. The clear front-runner today is opposition leader Lee Jae-myung, Yoon’s political archnemesis who led the impeachment push.

More than 61% of South Koreans want to see Yoon’s impeachment upheld, with about 37% for rejection, according to a recent poll.

Former South Korean President Park Geun-hye, who was removed early from office in 2017 over an influence-peddling scandal, saw her impeachment and criminal cases unfold simultaneously. But she wasn’t detained until the Constitutional Court upheld her impeachment. Her criminal trial then took more than a year to unfold.

Three top South Korean military officials—including Yoon’s former defense minister—have recently been indicted over the martial-law order for insurrection, among other charges. Several other top officials have been brought in for questioning.

Write to Timothy W. Martin at Timothy.Martin@wsj.com and Dasl Yoon at dasl.yoon@wsj.com