Former Defense Minister Kim Yong-hyun was arrested on suspicion of insurrection by advising conservative President Yoon Suk Yeol to declare martial law, according to the prosecution on Sunday. He is the first person to be arrested in connection with the six-hour martial law crisis in South Korea.

Investigative authorities in South Korea -- police and the prosecution -- are racing to look into Kim's case. The main opposition party, which has long been at loggerheads with the prosecution, expressed regret that the prosecution has an upper hand in the investigation into Kim through his arrest.

The prosecution's special investigation headquarters put Kim under emergency arrest without a warrant and confiscated his phone, as the 65-year-old is suspected of instigating violence to subvert national power, abusing his authority and obstructing others from exercising their rights.

The prosecution will have to seek a warrant to continue his detention within two days after the suspect was put under emergency arrest, which lasts up to 48 hours. Kim was incarcerated at Seoul Dongbu Detention Center.

The prosecution, which runs a special team of over 60 investigators, prosecutors and military prosecutors combined, unveiled plans to question Kim Sunday afternoon, and said it has initiated a probe into Yoon.

Kim voluntarily appeared for questioning at the Seoul Central District Prosecutors' Office at around 1:30 a.m. Sunday.

Hours after Kim's emergency arrest, police announced they had raided Kim's official residence and his office in the Defense Ministry. As of Sunday afternoon, some 150 police officers were dispatched to a special team to look into South Korean officials' insurrection allegations under the police's National Office of Investigation.

Police were also looking to secure the phone call records through a warrant.

This comes as 14 lawmakers from the Democratic Party, the Rebuilding Korea Party and the Basic Income Party combined claimed Sunday at a press briefing at 11 a.m. that police's procrastination caused the prosecution to take control of the investigation into Kim's insurrection allegations, although police are the only valid authority to initiate the probe.

This came as members of the National Assembly's public administration and security service affiliated with the three opposition parties released a joint statement Sunday to call for a swift probe.

The lawmakers also argued that police have been slow to launch investigations into other major suspects of insurrection including Yoon.

"It is imperative that (the investigative authorities) stop the suspects from destroying evidence," read the joint statement.

Kim, along with other Cabinet members, offered to resign Wednesday as Yoon lifted his martial law declaration in the morning. Yoon, on Thursday, accepted the resignation of Kim, who was facing an impeachment vote and hearing at the National Assembly.

Kim is one of Yoon's high school alumni, along with Interior Minister Lee Sang-min and Defense Ministry's counterintelligence commander Yeo In-hyung, who were allegedly involved in the nation's first martial law declaration in four decades.

A flight ban was imposed on Kim on Thursday. Political parties and civic groups filed complaints with three investigative authorities: Police, prosecution and the independent Corruption Investigation Office for High-ranking Officials.