A long-time friend, the daughter of a cult leader, is accused of undue influence over the president.
Choi Soon-sil is alleged to have pushed businesses to donate millions of dollars to foundations she controlled, helped choose presidential aides, and even picked the president's clothes.
Ms Park apologised on TV on Friday.
Ms Park was close to tears as she addressed the nation, and said the scandal involving her confidante Ms Choi was "all my fault". The president admitted she had let Ms Choi edit her speeches.