The questioning of Choi Soon-sil, whom Park has described as a life-long friend, came ahead of a ninth straight weekend rally in central Seoul demanding the immediate ouster of Park.

Choi and other former presidential aides were charged in November with abuse of power and fraud, but Park has immunity from prosecution as long as she is in office even though her powers are suspended.

"The charges in the indictment are but a very small part of the 14 points under investigation by the special prosecutor," said Lee Kyu-chul, a spokesman for the team of investigators probing the scandal.

Choi will be questioned on charges of bribery and transferring embezzled assets abroad, Lee told a briefing.

Choi, wearing a grey prison uniform and a surgical mask, was taken to the special prosecutor's office from detention, pushed by a throng of correctional officers through a media scrum.

She did not answer journalists' questions about the charges.

The special prosecutor has up to 100 days to investigate allegations that Park colluded with Choi and her aides to pressure big conglomerates to contribute 77 billion won ($64 million) to foundations set up to back her policy initiatives.

Park has denied wrongdoing but apologised for carelessness in her ties with Choi.

The friendship dates back to the 1970s when Park served as acting first lady after her mother was killed by an assassin's bullet intended for her father, then-president Park Chung-hee.

Five years later, in 1979, Park's father was murdered by his disgruntled spy chief.

Park's impeachment is being reviewed by the Constitutional Court which has up to 180 days from the day of the Dec. 9 impeachment to decide whether to uphold it or reinstate Park.