The arrest process could last until Monday.

"We have begun to execute an arrest warrant for President Yoon," the Corruption Investigation Office for Senior Officials (CIO) said in a press release.

The CIO has until Monday to issue an arrest warrant for Mr Yoon on charges of rebellion and abuse of power over his brief imposition of martial law on December 3.

However, protests by Mr. Yoon's supporters outside the Presidential Palace have complicated the CIO's efforts, along with the possibility of conflict with the Presidential Security Service.

Thousands of supporters have gathered near the building in recent days to protest Yoon’s impeachment and prevent his arrest. Some have been dispersed or taken away by police.

Carrying out the arrest warrant on Saturday or Sunday could risk facing larger crowds, while carrying it out on Monday would be too close to the deadline, observers said.

The CIO has been working with the police and the Ministry of National Defense’s investigative unit to conduct a joint investigation into Yoon’s martial law. The CIO executed the arrest warrant after Yoon ignored all three summonses. The agency also obtained a search warrant for the presidential residence.

Yoon's defense team has called the arrest warrant "illegal and invalid" and has filed a motion to quash it.

In the event that the Presidential Security Service or Mr. Yoon's supporters attempt to prevent the CIO from carrying out the arrest warrant, the police have said they will arrest them on charges of obstruction of official duty. About 3,000 police officers have been mobilized to deal with the situation.

Protesters holding American flags

Many protesters held signs protesting the arrest of South Korea's impeached president, as well as carrying American flags in the hope that the incoming administration of President-elect Donald Trump will intervene to help Mr. Yoon escape charges.

Protester Pyeong In-su, 74, said police had to be stopped by “patriotic citizens,” a term Mr Yoon used to describe pro-democracy protesters gathering near his residence.

Holding an American and South Korean flag bearing the words "Let's Go Together" in English and Korean, Pyeong said he hoped incoming US President Donald Trump would come to Yoon's aid.

“I hope after Trump's inauguration, he can use his influence to help get our country back on track,” he said.

If Mr. Yoon is arrested, investigators plan to take him to the CIO headquarters in Gwacheon, south of Seoul, for questioning before detaining him at the Seoul Detention Center in nearby Uiwang. After his arrest, the CIO will have 48 hours to either request a formal arrest warrant or release Mr. Yoon.

Mr. Yoon has been isolated since he was impeached and suspended from office on December 14. In addition to the criminal investigation, his impeachment is now before the Constitutional Court, which will decide whether to reinstate or permanently remove him from office. A second hearing in that case is scheduled for later Friday.

Hoang Anh (according to Yonhap, KOT, Reuters)