Park arrived at a prosecutors' office for questioning on Tuesday over allegations she colluded with a longtime confidante to extort money from businesses. There were indications the questioning could go on until midnight local time.
"I will undergo the investigation sincerely," she said, apologizing to the public for the scandal which ruined her presidency.
Having lost presidential immunity, Park could soon face criminal charges connected to bribery, extortion and abuse of power in connection with allegations of conspiring with her friend, Choi Soon-sil. Choi, as well as the head of Samsung are already facing criminal charges in connection with the allegations facing Park.
Lengthy jail term possible
Park has denied wrongdoing, but could face more than 10 years in jail if convicted of receiving bribes from bosses of big conglomerates.
South Korea's first female president, Park was impeached by parliament in December. Her dismissal was confirmed by the country's top court earlier this month, ending the political career of a woman who grew up in the presidential palace as the daughter of army-backed dictator Park Chung-Hee.
The March 10 ruling made Park South Korea's first democratically elected leader to be forced out of office since democracy replaced dictatorship in the late 1980s.
Late last year, millions of people took to the streets to demand her removal over the sprawling scandal.
Following Park's impeachment, South Korea will hold a new presidential election on May 9.
mm/gsw (AFP, AP, dpa, Reuters)