Park's approval ratings have hit a historic low of five percent -- a record for a sitting president -- over the scandal involving her close friend Choi Soon-Sil.
Choi has been arrested for fraud and also stands accused of meddling in state affairs -- including government appointments and policy decisions -- despite holding no official position.
Ahn Jong-beom, a former senior advisor to Park, was formally arrested early Sunday on charges of abuse of power and attempted coercion, the Yonhap news agency reported.
Ahn, who has been in custody since Wednesday after stepping down late last month said he would "take responsibility" for failing to properly advise the president, Yonhap reported.
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Prosecutors also arrested Jeong Ho-Seong, another former presidential aide, over allegations that he leaked classified information.
The 47-year-old Jeong, who was known as Park's right hand man and has assisted her since 1998, is suspected of passing presidential speeches and official documents to Choi.
But the reshuffle has done little to assuage public anger, with tens of thousands of protesters rallying to demand her resignation in a candlelight demonstration yesterday.
Police said around 50,000 people had turned out for the rally -- more than double the size of a similar anti-Park protest the week before. Organisers said the number was closer to 200,000.
Han Gwang-Ok, Park's new chief of staff who was appointed last week, said Saturday's protest reflected the gravity of the current situation.
In a televised address Friday, Park agreed to be questioned by prosecutors, and sought to portray herself as an over-trusting friend who had let her guard down at a moment of weakness.
Her voice choking with emotion, Park said she had been living a "lonely life" as president and had turned to Choi for company and help.
The South Korean media has portrayed Choi, whose late father was a shadowy religious leader and an important mentor to Park, as a Rasputin-like figure who wielded an unhealthy influence over the president.