The main opposition Liberty Korea Party (LKP) began boycotting the regular National Assembly session on Saturday in protest against what it claimed is the liberal government's bid to control public broadcasters.
The measure came after a district court issued an arrest warrant for MBC President Kim Jang-gyeom, who is accused of unfair labor practices.
Earlier, the broadcaster's labor union voted to strike from Monday, saying the strike would continue until Kim stepped down.
The union members claim that Kim, appointed under the previous Park Geun-hye administration, hurt the freedom of journalism by demanding favorable reports about conservatives.
The union members claim that journalists who opposed the tone of the reports were isolated from news reporting and even fired.
At a press briefing on Saturday, the LKP defined the moves as "a leftist populist government's initiative toward dictatorship."
"We decided to boycott all National Assembly schedules, including the plenary session, ongoing confirmation hearings, and a special committee to revise the Constitution."
On Friday, the Assembly began its first regular session after President Moon took office in May. Parties have agreed to endorse Kim Yi-su, the nominee for president of the Constitutional Court, at the upcoming plenary session slated for Monday.
Theoretically, four other parties besides the LKP are able to vote to pass Kim, the decision having been up in the air since he was tapped in May. This could be a political burden for President Moon.
The ruling Democratic Party of Korea (DPK) lashed out at the LKP, citing the ex-President Lee's attempts to dominate the "left-leaning" MBC in 2008.
"The LKP's reaction proves that the party is part of the old evils who made the public broadcasters a mouthpiece of the conservative government," DPK vice spokeswoman Kim Hyo-eun said Saturday.
The minor liberal People's Party also blasted the LKP, saying, "The boycott is unjustifiable. The LKP's claim against the judiciary's enforcement will not gain public support."
But the conservative Bareun Party criticized Cheong Wa Dae's "blatant" measures against media companies, but stayed silent on the boycott.
Following the liberal president's inauguration, the fired journalists are returning to the stage, with three YTN reporters returning to their workplace nine years after being dismissed.
From Monday, journalists at KBS, the nation's largest public broadcaster, will go on strike, vowing to fight until CEO Ko Dae-young steps down.
President Moon has criticized the "alliance" between conservatives and the media. In the March televised debate on MBC, he said, "The MBC has collapsed severely. Citizens have been calling for rooting out the old evils and the most urgent task is the media."
Moon said during the debate, "If public broadcasters had performed their duty, the situation in which President Park was impeached and put behind bars would never have happened (as they would have discovered any misdeeds.)"