SEOUL, South Korea — President Park Geun-hye acted as a criminal accomplice for her longtime friend in extorting tens of millions of dollars from businesses, prosecutors said on Sunday, dealing another blow to the South Korean leader, who faced a nationwide call to step down.

Under the country’s Constitution, prosecutors cannot indict the president on criminal charges until after she leaves office. But their findings further weakened Ms. Park’s authority, and prompted opposition leaders, as well as some members of Ms. Park’s governing party, to demand on Sunday that Parliament start a legal process to impeach her. On Saturday, huge crowds protested in Seoul and other major cities for the fourth straight weekend, demanding that she resign or face impeachment.

Prosecutors identified Ms. Park as an accomplice and a “criminal suspect” on Sunday while indicting her friend, Choi Soon-sil, on criminal charges including extortion and abuse of official power.

A former presidential aide to Ms. Park was also indicted on Sunday, on the same charges, as prosecutors accused him of helping Ms. Choi with the extortion. Another former presidential aide was indicted on charges of leaking confidential government documents to Ms. Choi. Although she had no experience in government or policy making, Ms. Choi was accused of meddling in a wide range of state affairs, like editing Ms. Park’s speeches and helping her appoint key officials.