TOKYO — South Korean President Park Geun-hye suffered a heavy blow Sunday, when prosecutors indicted her friend on charges including extortion and abuse of power, and indicated they thought the president was complicit in the crimes.

The prosecution said it would continue to try to question Park, with the announcement effectively making her a suspect rather than a witness, while opposition leaders said they would redouble their efforts to force her out of office.

“There are now sufficient grounds for her impeachment,” Moon Jae-in, a presidential hopeful, said.

It came a day after hundreds of thousands of South Koreans took to the streets of central Seoul for a fourth consecutive Saturday, calling on Park to resign. The demonstrations are the largest since South Korea democratized in 1987.

The prosecution said Sunday that it had indicted Choi Soon-sil, Park’s friend of 40 years who held no official position, with abuse of power, coercion, attempted coercion and fraud. It also indicted two former presidential secretaries on charges including abuse of power, attempted coercion, fraud and divulging classified information.

The charges come out of a corruption and influence-peddling scandal that has Park, South Korea’s first woman president, fighting for her political life.