Prosecutors questioned two key witnesses as part of ongoing investigations into Ulsan mayoral election meddling scandal, allegedly involving President Moon Jae-in's secretaries and police, in Seoul on Monday.

Im Dong-ho, a former senior member of the ruling Democratic Party, and former Ulsan Mayor Kim Gi-hyeon were summoned to the Seoul Central District Prosecutors Office in the afternoon for questioning about the allegations that Song Byung-gi, Ulsan's vice mayor for economic affairs, conspired with several presidential aides and police to influence the city's mayoral poll last year in favor of DP candidate Song Cheol-ho.

Im and Kim were previously grilled by prosecutors twice each.

Prosecutors last week requested an arrest warrant for Song Byung-gi, known as the source who tipped a former presidential secretary off about bribery allegations involving Kim's confidants.

Kim, who was then seeking reelection as mayor in the June 2018 local elections as a candidate of the main opposition Liberty Korea Party, lost the race to current mayor Song Cheol-ho, a longtime friend of President Moon.

The case blew up into a political scandal after Kim publicly claimed that the Ulsan Metropolitan Police Agency deliberately raided his office a few months ahead of the election with the presidential office behind the move.

Kim and his confidants were acquitted by a local court after the election ended, while Cheong Wa Dae, the presidential office, has flatly denied all allegations of election meddling.

Im has drawn media attention, as he allegedly gave up competing against Song Cheol-ho in the DP's race to pick its Ulsan mayoral candidate, in return for a ranking government post.

Prosecutors suspect that Cheong Wa Dae officials had proposed a post of consul general in Japan to Im in return for giving up his Ulsan mayoral bid.

But Im has denied the allegations of a government post proposal by Cheong Wa Dae, arguing he "only privately" discussed the matter with Han Byung-do, former senior secretary for political affairs.

In controversial remarks, Im reportedly told prosecutors in his previous questioning that he saw a memo from Song Byung-gi's diary quoting the vice mayor as telling Cho Kuk, former senior presidential secretary for civil affairs, he has a card to force Im to quit.

"I was never offered a ranking government post in exchange for not running for the DP's Ulsan mayoral candidate race," Im said before entering the Seoul prosecutors' office, denying allegations of Cheong Wa Dae's election intervention.

He also claimed vice mayor Song's memos are not reliable. (Yonhap)