SEOUL -- South Korea's police chief became the latest top official to be arrested, Yonhap news agency said on Wednesday, in a widening investigation into President Yoon Suk Yeol's failed martial law that has plunged the country into a constitutional crisis.

The main opposition Democratic Party (DP) said on Wednesday it plans to hold a parliament vote to impeach Yoon on Saturday, with some members of the president's People Power Party (PPP) having spoken out in favor of such a motion.

"The impeachment train has left the platform. There is going to be no way to stop it," DP leader Lee Jae-myung said at the start of a party meeting.

The first impeachment vote last Saturday failed as most PPP members boycotted the session.

Yoon's surprise martial law declaration stunned the country and plunged Asia's fourth-largest economy and a major U.S. ally into a leadership crisis, sending shockwaves through diplomatic and economic fronts.

National Police Commissioner Cho Ji-ho was arrested early on Wednesday on insurrection charges, Yonhap said. Cho is accused of deploying police to block lawmakers from entering parliament after Yoon declared martial law on December 3.

Soon after Yoon's surprise late-night declaration, lawmakers including some members of his own party defied the security cordon around parliament and voted to demand the president immediately rescind martial law, which he did hours later.

Yoon himself is now the subject of criminal investigation on insurrection charges but he has not been arrested or questioned by authorities.

He has apologized but has not responded to growing calls for him to step down, even from some members of his own party.

After appearing on live television on Saturday to apologize, Yoon has not been seen in public. PPP leader Han Dong-hoon said Prime Minister Han Duck-soo would manage state affairs while the party looks for an "orderly" way for the president to resign.

The constitutional legitimacy of that has been questioned by opposition parties and some legal scholars.

Yoon's office said on Tuesday it had "no official position" when asked who was running the country.

Kwak Jong-geun, the commander of the Army Special Warfare Command, told a parliament committee on Tuesday that Yoon had ordered him to send his troops to parliament on December 3, "break the door down" and "drag out" lawmakers.

Yoon's then-defense minister, Kim Yong-hyun, has also been accused by military officers of issuing the same order. Kim has resigned and has been arrested.

Parliament is scheduled to hold a session on Wednesday to introduce a bill to impeach Yoon. A two-thirds majority of the opposition-controlled unicameral assembly is needed to pass the bill. The Constitutional Court then deliberates the case and decides whether to remove the president from office.

The country's metal workers' union, including workers at the Kia Corp (000270.KS) automakers, has declared a protest strike for Wednesday. Members of financial institutions including the Bank of Korea plan to join a protest rally on Wednesday.