Mr Roh made a public apology for the corruption scandal
South Korea's former president is being questioned over allegations that he took millions of dollars in bribes from a wealthy businessman.
Prosecutors have to decide whether to charge Roh Moo-hyun, who came to power in 2003 promising his administration would be free from foul play.
Before leaving his rural hometown for Seoul, he apologised for the scandal.
"I feel ashamed before my fellow citizens. I am sorry for disappointing you," he said in a televised statement.
Mr Roh was taken by police escort to the Supreme Public Prosecutors Office in the capital.
Questioning is expected to last late into Thursday or even early Friday.
The former president, who left office last year, faces accusations that he took millions of dollars in bribes from the wealthy head of a shoe manufacturing company.
At one point a bag containing the equivalent of $1m (£670,000) in cash is said to have been delivered to the presidential office.
In a statement posted on his website earlier this month, Mr Roh admitted that his wife received a substantial sum of money from the businessman, but suggested it was not a bribe, but a payment to help her settle a debt.
The BBC's John Sudworth in Seoul says that for the South Korean public, the allegations are a reminder of an era many hoped had passed, in which former presidents found themselves embroiled in corruption scandals, and sometimes convicted and jailed for their excesses.
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