Earlier in the day, Hwang rejected the independent counsel's request to extend his legal term, which ends Tuesday. The four opposition parties also vowed to convene an extraordinary sitting to enact yet another special probe to examine the scandal.

As the three opposition parties hold 166 seats in the 300-seat National Assembly, pundits said the parties will face no troubles in securing the minimum vote to impeach the prime minister.

Rep. Choo Mi-ae of the main opposition called Hwang's rejection "the worst decision in the republic's history."

"It will be remembered as a decision that goes against the rule of law in favor of criminal acts by President Park Geun-hye and chaebol," Choo said.

Rep. Oh Shin-hwan of the Bareun Party said the decision contradicts the public's hope to find the truth behind the corruption scandal, calling the acting president's move an act of "tyranny."

The party spokesman warned that the public will not sit idle if the decision is based on his potential presidential ambitions.

Since Park's impeachment in December, Hwang has been viewed as a potential presidential candidate for the conservative bloc, although he has refused to make his true intentions known.

The People's Party echoed the view, claiming Hwang is only struggling to shield Park despite the public's calls for a thorough investigation.

The Bareun Party said although it is disappointed by the acting president's decision, the act does not constitute grounds for an impeachment. The party, however, said it will support adopting another special probe into the case.

The ruling Liberty Korea Party, on the other hand, said the opposition parties' plan to impeach the acting president is an "unconstitutional and irrational" action.

"The idea is unconstitutional in that the acting president was designated in accordance with the Constitution," Rep. Kim Sung-won of the ruling party said, claiming the ouster of the prime minister will bring more confusion to the country already rocked by the scandal.

"Hwang carried out state affairs in silence despite challenges.

He made the decision on the independent counsel issue, taking into account the related law and principles," Kim said.

"The opposition parties should cooperate in normalizing state affairs and stop violating the Constitution in order to satisfy their own political interests." (Yonhap)