South Korean prosecutors are likely to question President Park Geun-hye over suspicion that she let a shadowy longtime confidante manipulate power from behind the scenes, state-run Yonhap news agency reported Sunday.

If would be the first time that a sitting president has been questioned by prosecutors. The explosive scandal is the most serious challenge for Park, whose public apologies have done little to calm public anger.

Yonhap quoted a Seoul Central District Prosecutors' Office official as saying that the questioning is most likely to take place later this week. The official was not named.

Yonhap Korean-language service quoted an unidentified prosecution official as saying prosecutors want to question Park face-to-face on either Tuesday or Wednesday at the latest. The report said that prosecutors conveyed their position to Park's office and were awaiting a response.

The presidential Blue House had no immediate comment on the Yonhap report.

In addition to allegedly manipulating power, the president's confidante, Choi Soon-sil, the daughter of a late cult leader who emerged as Park's mentor in the 1970s, is also suspected of exploiting her presidential ties to bully companies into donating tens of millions of dollars to foundations she controlled.

Choi was formally arrested on Nov. 3 on charges of fraud and abuse of power. Prosecutors have until Nov. 20 to formally charge her.

On Saturday, hundreds of thousands of protesters flooded Seoul's streets to demand Park's resignation in what may be South Korea's largest protest since it shook off dictatorship three decades ago.

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In an attempt to stabilize the situation, Park said Tuesday that she would let the opposition-controlled parliament choose her prime minister. But opposition parties say her words are meaningless without specific promises about transferring much of her presidential powers to a new No. 2.

Under South Korean law, the president has immunity from prosecution except in cases of treason, but she can be investigated.

Park has 15 months left in her term. If she steps down before the end of it, an election must be held within 60 days.