The main opposition Democratic United Party on Friday intensified its attacks on President Lee Myung-bak, following the disclosure of documents alleging that his government carried out extensive spying on not only public officials but also civilians years ago.

The ruling Saenuri Party also condemned the alleged surveillance and called for a thorough probe in an apparent attempt to distance itself from the scandal before next month's general elections.

Striking journalists at public broadcaster KBS reported in a news program posted on the Internet on Thursday that they obtained

2,619 cases of government reports that an ethics team under the Prime Minister's Office drew up in 2008-2010 after spying on top officials, politicians, journalists and civilians.

According to the report, the documents showed that the team, which is tasked with ensuring public officials are not involved in corruption, recklessly collected information on not only government officials, but also civilians considered critical of the government, such as labor union leaders and reporters at unfriendly media firms.

The revelations added to the already snowballing scandal surrounding the government's illegal surveillance of a businessman critical of President Lee and the presidential office's alleged attempt to cover it up. The opposition party has called the case a Korean version of the U.S. "Watergate scandal."

The politically sensitive case is expected to be a hot-button issue in the run-up to the April 11 parliamentary elections.

"The reported truth about illegal surveillance is shocking," said opposition party leader Han Myeong-sook. "The problem is that the results of the surveillance must have been reported to the VIP," she said, urging President Lee to clarify whether he was involved and punish those responsible.

Rep. Park Young-sun, a ranking DUP lawmaker, claimed that the fresh revelations are only "the tip of the iceberg."

"We have defined this case as a Korean version of the Watergate scandal," she said, claiming that it is time for the country to talk about Lee's resignation.

The ruling Saenuri Party also expressed shock and called for a thorough investigation.

"If this is true, this is an infringement on human rights and destruction of democracy," said Lee Sang-il, a party spokesman.

"The prosecution should sternly punish those responsible. ... We will closely watch the prosecution's probe and study other measures if the investigation is deemed insufficient."

Comments from the presidential office were not immediately available.

Earlier this month, prosecutors reopened an investigation into the surveillance scandal after one of the officials involved in the surveillance claimed that the presidential office attempted to cover up the illegal operation.

Following the previous investigation, seven officials were indicted for the illegal surveillance of the businessman, who posted a video clip criticizing the Lee government for resuming U.S. beef imports in 2008. The probe also concluded that the presidential office was not involved. (Yonhap News)