Former South Korean president Lee Myung-bak reported to prosecutors for questioning in a corruption probe Wednesday, apologising for the controversy.
Allegations of corruption involving the 76-year-old's relatives and aides during his 2008-2013 presidential term have mounted in recent weeks as prosecutors investigate multiple cases of bribery amounting to millions of dollars.
The investigation into Lee means all four living former South Korean presidents have been convicted, charged, or embroiled in criminal inquiries.
"I stand here with a heavy heart," Lee said as he arrived at the prosecutors' office in Seoul, after a car journey from his home in the south of the capital covered live on television.
"I'm very sorry for causing concern to the people," he told reporters, adding that South Koreans' livelihoods were "difficult" and the security situation on the Korean peninsula was "dire".
Lee, accompanied by his defence lawyer, was received by a senior prosecutor and given tea before being ushered into room 1001, the same place his successor and ousted former president Park Geun-hye underwent marathon questioning.
He was to be interrogated by three prosecutors and Yonhap news agency said the questioning -- which will be videotaped -- was expected to last around 20 hours, with prosecutors having prepared around 120 pages of questions relating to some 20 alleged offences.
"We will treat the former president with dignity but we will conduct a through and transparent probe," a senior prosecutor told journalists.
Lee is expected to return home after the questioning, but prosecutors are thought likely to then ask a court for an arrest warrant.
Two of Lee's former aides have been arrested as part of the investigation and the homes and offices of his brothers raided.
He has previously denounced the inquiry as "political revenge" and said Wednesday he hoped it would be the "last time in history" that a former South Korean head of state was summoned for questioning by prosecutors.
"As a former president, I have a lot to say about this but I will spare my words," he said.
South Korean presidents have a tendency to end up in prison -- or meet untimely ends -- after their time in power, usually once their political rivals have moved into the presidential Blue House.
Park was ousted last year over a massive corruption scandal that emerged in 2016, and the verdict in her bribery and abuse of power trial is due next month, with prosecutors demanding 30 years in jail.
Lee's own predecessor, the liberal Roh Moo-hyun, committed suicide by jumping off a cliff after being questioned over corruption allegations in 2009.
After the South embraced democracy in the 1990s former dictator Chun Doo-Hwan and his friend and successor Roh Tae-Woo were handed death sentences for their involvement in a 1979 military coup and for receiving hundreds of millions of dollars in bribes from businesses.
Their sentences were later commuted to life in prison, and they were eventually pardoned and released after serving about two years each.