The president's close friend and two other former aides have been formally charged on suspicion of interfering with state affairs.
South Korean President Park Geun-Hye played a "considerable" role in the corruption scandal that has sparked nationwide fury, a special investigation team has concluded.
Prosecutors said "the president was involved as a conspirator in a considerable part of the criminal activities" by her longtime friend Choi Soon-sil and two other former presidential aides.
Choi and the two aides were formally charged by prosecutors on Sunday on suspicion of interfering with state affairs.
"The special investigation team concluded that based on the evidence secured to date, the president was in complicity with Choi Soon-sil, An Chong-bum and Jeong Ho-seong to a considerable degree," Lee Young-ryeol, chief prosecutor of the Seoul Central District Prosecutors' Office, said.
The aides were charged with using presidential ties to pressure companies into giving tens of millions of dollars to foundations Choi controls.
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.
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 the Korean constitution, a sitting president cannot be indicted unless on charges of treason but Lee said the prosecutors "will continue to investigate the president."
Park's lawyer Yoo Yeong-ha said the prosecutors' announcement was "deeply regrettable".
He termed the prosecutor's comments an "imagination" and reiterated that Park would not make herself available for questioning.
The scandal has sent Park's public approval rating plunging to five percent, the lowest for any sitting South Korean president.
The main opposition Democratic Party and the centrist People's Party said Park will face impeachment proceedings if she refuses to resign.
Park has pledged to cooperate in the investigation after publicly apologising to the nation twice, saying that the incident was a result of her "shortcomings". But she has resisted calls to resign over the scandals and pushed back on the prosecutors' plan to question her last week.
Hundreds of thousands of people have taken to the streets of Seoul in the fourth straight weekend of protests against Park. The demonstration on Saturday was the biggest the country has seen since the 1980s.