The main opposition party's leader demanded Tuesday that President Park Geun-hye apologize over allegations of outside pressure in a prosecution investigation into suspicions the state intelligence agency meddled in last year's presidential election.

Democratic Party leader Kim Han-gil issued the demand a day after a senior prosecutor leading the investigation into the scandal testified during a parliamentary meeting that his team has been under constant pressure not to aggressively pursue the probe.

The prosecutor, Yoon Seok-youl, was excluded from the team last week for allegedly failing to report to his bosses before pressing additional charges in the case and briefly detaining some National Intelligence Service (NIS) agents suspected of involvement.

The opposition party claimed Yoon's removal was an attempt to blunt the investigation.

DP leader Kim said Yoon should be reinstated as head of the investigation team. Kim also demanded that three top officials -- NIS Director Nam Jae-joon, Justice Minister Hwang Kyo-ahn and Seoul District Prosecutor's Office chief Cho Young-kon -- resign immediately.

"Much of the truth has been laid bare before the people," Kim said during a meeting of party lawmakers, accusing the government of attempting to use "whatever means possible" to cover up the intelligence agency's "organized meddling in the presidential election."

"Democracy and the people in the Republic of Korea are being ridiculed," Kim said.

The scandal centers on suspicions that NIS agents posted Internet comments ahead of December's presidential election in a suspected attempt to help the then ruling party candidate Park Geun-hye win the vote.

The prolonged scandal has been brought back into the spotlight after fresh allegations emerged last week that NIS agents posted more than 55,000 messages on Twitter and that members of the defense ministry's cyber warfare command conducted a similar online smear campaign.

The spy agency has claimed that the online activity was part of its anti-North Korea psychological warfare. Park has also denied any link to the scandal, saying she neither had any knowledge of the agency's alleged wrongdoing nor did she benefit from it. (Yonhap News)