South Korean President

The scandal has sparked nationwide fury, with hundreds of thousands taking to the streets to call for Park's resignation, providing a stark challenge to her authority.

"The president played a collusive role in a considerable portion of the criminal activities involving the (three) people," said Lee Young-Ryeol, who is leading a probe into the scandal.

Park's ally Choi Soon-Sil and one of the president's former aides were arrested earlier this month on charges of fraud, coercion and abuse of power. Another presidential aide was arrested for leaking confidential state documents.

Choi, 60, has been accused of using her personal ties with Park to meddle in state affairs and coerce local firms to "donate" tens of millions of dollars to dubious non-profit foundations she then used for personal gain.

Park faces allegations that she helped Choi extract money from the firms and ordered her aides to leak state documents to Choi, who has no official title or security clearance.

Under Seoul's constitution, the incumbent president cannot be charged with a criminal offence except insurrection or treason but she can still be probed by prosecutors and potentially charged after leaving office.

Lee acknowledged that prosecutors could not formally charge Park for now but vowed to continue to investigate her.

Approval rating falls

Park earlier promised to answer prosecutors' questions -- making her the first South Korean president to be quizzed by prosecutors while in office.

More than 50 local firms including Samsung and Hyundai were forced to donate a total of 77.4 billion won ($65.5 million) to the two foundations controlled by Choi.

Many made the donations due to fear of political reprisals, such as harsh tax audits or difficulties getting regulatory approvals for their businesses, Lee said.

Choi also pressured major firms including the South's largest carmaker Hyundai and the top steelmaker Posco to award lucrative contracts to firms linked to Choi, he added.

One of the aides leaked 180 confidential documents to Choi, which included papers on foreign policy and nominations of top officials and cabinet members, according to prosecutors.

Choi is known to have wielded huge influence on the country's sports sector, including the nomination of officials and even preparations for the 2018 Winter Olympics.

Kim Chong, who served as vice sports minister for three years until last month, has also accused of helping Choi's foundations win lucrative state contracts.

Prosecutors are investigating whether Kim played a role in a recent decision by his ministry to provide a cash subsidy to a winter sports foundation run by Choi's niece, who is widely seen as her key aide.

The scandal has sent Park's public approval rating plunging to five percent -- the lowest for any sitting South Korean president.

Choi is a daughter of a shadowy religious figure who was a longtime mentor to Park until his death in 1994. He is known to have befriended Park after the assassination of her first lady mother in 1974.