Prosecutors on Monday raided the headquarters of the main opposition Democratic Party as part of an investigation into bribery allegations involving a close confidant of party leader Lee Jae-myung.

The Seoul Central Prosecutors Office is probing Kim Yong, deputy head of the Democratic Party-affiliated Institute for Democracy think tank, on suspicion of receiving a total of 847 million won ($591,000) from property developers in the city of Seongnam, just south of Seoul, in violation of the political fund law.

Amid intense resistance and protest by party officials and lawmakers, prosecutors searched Kim's office in the Democratic Party headquarters to seize relevant documents. It was the second attempt to raid the Democratic Party headquarters after an earlier attempt fell through due to protests from party officials.

The Democratic Party has denounced the investigation as a political reprisal and an attempt to oppress the party.

Lee denounced the raid as "violence" against the opposition.

"It is hard to understand from common sense that a raid was conducted ahead of the president's parliamentary budget speech," he said. "It comes to my mind that politics are gone and all that's left is domination."

Lee also renewed his call for an independent counsels probe into the case.

Kim, who was arrested over the weekend, is under suspicion of taking the funds between April and August last year ahead of the party's primary election to pick the candidate to run for this year's presidential election.

Prosecutors suspect the money was used to help fund Lee's campaign as Kim served as a senior aide in charge of fundraising on the primary election campaign for Lee at the time.

Lee won the party's presidential nomination but was defeated by a narrow margin by President Yoon Suk-yeol in the March election.

The real estate developers in question are key suspects in a corruption case involving a lucrative housing development project in Seongnam's Daejang-dong district that is currently under a prosecution probe. Lee was Seongnam mayor when the project was launched in 2015.

After the prosecution's first attempt to raid the Democratic Party headquarters last week, Lee strongly denounced the investigation and claimed he is innocent.

Prosecutors are expected to expand the investigation to determine whether any illegal political funds were used in Lee's 2014 run for the Seongnam mayoralty or any other elections. (Yonhap)