Authorities entered the compound of impeached South Korean President Yoon Suk on Friday to execute an unprecedented arrest warrant, avoiding crowds of protesters outside but facing presidential security forces inside.

It was not clear whether the Presidential Security Service (PSS), which blocked investigators with a search warrant for Yoon’s office and official residence, would try to stop his arrest.

Yoon is under criminal investigation over the December 3 martial law effort that stunned South Korea, Asia’s fourth-largest economy and one of the region’s most vibrant democracies.

The arrest would be the first of a South Korean president.

Reuters witnesses said officers from the Corruption Investigation Office (CIO), which leads a joint investigation team including police and prosecutors, arrived at the gate of Yoon’s compound shortly after 7 a.m. (2200 GMT) on Thursday.

The CIO vehicles did not immediately enter the compound because a bus was blocking the driveway.

Some CIO officers then drove through an open gate and passed the bus, but another bus and an armored vehicle passed through the driveway shortly before they were taken away.

After entering the compound, the CIO and accompanying police were surrounded by PSS officers, and military officers were also assigned to presidential security, media reported. South Korea’s Ministry of National Defense said the military is under PSS control.

Yonhap news agency quoted the head of the PSS as saying that authorities were not authorized to search Yoon’s home.

The clash lasted until 10 p.m.

Yoon’s lawyer issued a statement Friday saying that the execution of an invalid arrest warrant against Yoon was illegal and that they would take legal action without elaborating.

Protesters gathered outside the house in the morning amid media reports that investigating authorities would quickly try to execute the arrest warrant approved Tuesday after Yoon refused a summons to appear.

Yoon’s lawyers have said the arrest warrant is illegal and invalid because the CIO does not have the authority to seek a warrant under South Korean law.

Yoon has been in solitary confinement since he was impeached and removed from office on December 14.

Separate from the criminal investigation, his impeachment case is currently before the Constitutional Court, which will decide whether to reinstate him or permanently remove him.