Ousted South Korean President Park Geun-hye is being questioned by prosecutors over a corruption scandal that brought her down.
After arriving at the office, she told reporters she was “sorry”.
Ms Park resisted efforts to question her when she was president, but lost her immunity when judges upheld parliament’s decision to impeach her.
She could be charged for allegedly allowing close friend Choi Soon-sil to extort money from large firms.
Ms Choi has been charged with bribery and corruption.
Prosecutors are expected to hold Ms Park for a marathon questioning session that will last till late in the night. One of her lawyers said a doctor was doing check-ups during breaks “as her health isn’t looking well”.
On Tuesday, Ms Park’s supporters gathered outside her home in an affluent suburb of Seoul, as she was escorted by police to the prosecutors’ office in a short journey covered live on television.
People waved the South Korean flag, a symbol of the pro-Park movement.
“I am sorry to the people. I will faithfully cooperate with questioning,” Ms Park told the media when she arrived.
Ms Park is the first democratically elected leader to be ousted in South Korea.
Thousands of people celebrated in Seoul after her removal from office on 10 March. However, there were also angry protests by her supporters outside the Constitutional Court.
The court ruling was the culmination of months of political turmoil and public protest. An election will now be held by 9 May.
Hwang Kyo-ahn, who is loyal to Ms Park, is currently the acting president.
Ms Choi is accused of using her presidential connections to pressure companies to give millions of dollars in donations to non-profit foundations she controlled.
Ms Park, 65, is alleged to have been personally involved in this, and to have given Ms Choi unacceptable levels of access to official documents.
Parliament voted to impeach Ms Park in December.
On 10 March, the Constitutional Court ruled that Ms Park’s actions “seriously impaired the spirit of… democracy and the rule of law”.
Judges said she had broken the law by allowing Ms Choi to meddle in state affairs, and had breached guidelines on official secrets by leaking numerous documents.
Ms Park had “concealed completely Choi’s meddling in state affairs and denied it whenever suspicions over the act emerged and even criticised those who raised the suspicions,” the ruling said.
Prosecutors are now questioning Ms Park in what could be a lengthy days-long process.
They had previously accused Ms Park of colluding with Ms Choi, which Ms Park has strenuously denied. She had also previously refused to take part in investigations.
But now that she has lost her presidential immunity, she could be charged for abuse of power and coercion to bribery.