The ruling Democratic Party called Wednesday for a thorough investigation of former President Lee Myung-bak, while the main opposition Liberty Korea Party denounced the probe as political revenge ahead of June's local elections.

Lee, president from 2008-2013, appeared at the Seoul Central District Prosecutors' Office for questioning over a string of bribery and other corruption charges, becoming the country's fifth former president to undergo questioning by prosecutors.

"I hope the prosecution will conduct a thorough investigation in accordance with law and principles so as not to leave an ounce of suspicion," Rep. Choo Mi-ae, head of the ruling party, said during a meeting of the party's supreme council.

Choo said the fact that Lee is facing about 20 counts of charges of influence-peddling and other crimes makes it serious enough to be listed in the Guinness World Records. She also rejected as groundless Lee's claims the investigation is political retaliation.

Floor leader Woo Won-shik also criticized Lee for renewing the claim of political revenge, saying he "would have nothing to say even if he had 10 mouths."

Woo said Lee should extend words of apology and show an apologetic attitude if he knew how people feel about him.

The main opposition LKP, however, renewed Lee's political revenge claim, accusing the administration of President Moon Jae-in of using the investigation for political gains ahead of June's gubernatorial and mayoral elections.

"Did they have to force a former president to stand in the photo line (at the prosecutors' office) out of pure determination to get revenge?" Hong Joon-pyo, head of the party, said in a Facebook posting. "It will come back as a boomerang."

The party's floor leader, Rep. Kim Sung-tae, also said it is a historical tragedy for the former president to undergo questioning by prosecutors and hopes the politics of revenge will no longer be repeated.

The minor opposition parties called for a thorough investigation, but they also stressed the need for revising the Constitution to change what they called the "imperialistic" presidential system that has long been accused of concentrating too much power in the president in a way prone to corruption.

"Former President Lee is the worst one can be when it comes to corruption, and we have to bring his crimes to light through a strict investigation, and the corresponding maximum legal penalty should be given," said Rep. Kim Dong-cheol, floor leader of the minor opposition Bareunmirae Party.

Kim also pointed out that the current presidential system is the "root" of evil and called for a constitutional revision.

"This is not a liberal or conservative issue, but this is a problem of the system," he said.

Rep. Cho Bae-sook, head of the Party for Democracy and Peace, also called for a constitutional revision.

"It is because of the imperialistic presidential system that a president ends up being a criminal after wielding absolute power," she said. "A revision of the Constitution must be made in a way that dismantles the presidential system."

Cho also said it is tragic for a former president to appear at a prosecutors' office for questioning, but anyone should be punished if they committed crimes no matter how high they are.

Her party also issued a statement urging Lee to come clean on his wrongdoing.

"A president, who abandoned public interests for the sake of private interests and who abandoned the rule of law and committed crimes, must be sternly punished," Rep. Choi Gyung-hwan, spokesman of the party, said in a statement. (Yonhap)