South Korean investigators entered the residence of impeached President Yoon Suk Yeol and began executing a warrant for his arrest on Friday over his failed martial law bid, marking the first time the country has ever sought to arrest a sitting leader.

The suspended president, who issued a bungled declaration on 3 December that shook the vibrant East Asian democracy and briefly sent it back to the dark days of military rule, now faces arrest, imprisonment, or, at worst, the death penalty.

"The execution of the arrest warrant for President Yoon Suk Yeol has begun," said the Corruption Investigation Office (CIO), which is investigating Yoon's short-lived martial law declaration. CIO officials and police were seen entering the president's residence.

Yoon, already suspended from duty by lawmakers, would become the first sitting president in South Korean history to be arrested.

CIO investigators, including senior prosecutor Lee Dae-hwan, were allowed through heavy security barricades to enter the residence and attempt to execute the warrant to detain Yoon, AFP reporters observed.

However, they were "blocked by a military unit inside" after entering, Yonhap news agency reported.

It had been unclear whether the Presidential Security Service, which still protects Yoon as the sitting head of state, would comply with investigators' warrants.

Members of his security team have previously blocked police raids of the presidential residence, but it was not immediately clear which unit had blocked investigators on Friday.

Yoon's legal team decried the attempt to execute the arrest warrant, vowing to take further legal action against the move.

"The execution of a warrant that is illegal and invalid is indeed not lawful," Yoon's lawyer Yoon Kap-keun said.

Dozens of police buses and hundreds of uniformed officers lined the street outside the compound in central Seoul, AFP reporters saw.

Some 2,700 police and 135 police buses have been deployed to the area to prevent clashes, Yonhap reported, after Yoon's supporters clashed with anti-Yoon demonstrators Thursday.

Yoon has been holed up inside the residence since a court approved the warrant to detain him earlier this week, vowing to "fight" authorities seeking to question him over his failed martial law bid.

All-Night Prayers

South Korean media have reported that CIO officials want to arrest Yoon and take him to their office in Gwacheon near Seoul for questioning.

After that, he can be held for up to 48 hours under the existing warrant. Investigators would need to apply for another arrest warrant to keep him in custody.

Following chaotic protests Thursday, a handful of Yoon's die-hard supporters, including far-right YouTube personalities and evangelical Christian preachers, camped outside his compound overnight — some holding all-night prayer sessions.

"Illegal warrant is invalid," they chanted early Friday, as police and media gathered outside the residence.

"Yoon Suk Yeol, Yoon Suk Yeol," they shouted, waving red glow sticks.

Pro-Yoon protester Rhee Kang-san told AFP many were "rooting for the president" to survive the arrest attempt.

"We are sending him our wishes. Many of our colleagues stayed overnight to show their support. We will continue to do the same today, no matter what happens later," he said.

Yoon's lawyer confirmed to AFP Thursday that the impeached leader remained inside the presidential compound.

Yoon's legal team had already filed for an injunction with the Constitutional Court to block the warrant, calling the arrest order "an unlawful and invalid act," and also submitted an objection to the Seoul court that issued it.

But the head of the CIO, Oh Dong-woon, has warned that anyone attempting to block authorities from arresting Yoon could themselves face prosecution.

Along with the summons, a Seoul court issued a search warrant for his official residence and other locations, a CIO official told AFP.

South Korean officials have previously failed to execute similar arrest warrants for lawmakers — in 2000 and 2004 — because party members and supporters blocked police for the seven days the warrants were valid.