Ousted South Korean President Park Geun-hye is being questioned by prosecutors over a corruption scandal that brought her down.
Earlier this month, judges upheld parliament’s decision to impeach Ms Park, who has now lost her immunity.
Ms Park’s supporters gathered outside her home in Seoul, as she was escorted by police to the prosecutors’ office.
Upon arrival, she apologised to the public, adding: “I will undergo the investigation sincerely.”
She could face criminal charges over accusations she allowed close friend Choi Soon-sil to extort money from firms in return for political favours.
She 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, angry protests by her supporters outside the Constitutional Court left two people dead.
The court ruling was the culmination of months of political turmoil and public protest. An election now will be held by 9 May.
Hwang Kyo-ahn, who is loyal to Ms Park, is now the acting president.
Why did Park lose her job?
At the heart of the drama lies the close friendship between Ms Park and Ms Choi.
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.