South Korean prosecutors said on Sunday that President Park Geun-hye was involved in "criminal activities" as they indicted a close friend and two former aides over a corruption scandal. 

Prosecutors are planning to question Park, who has immunity but can be investigated, Lee Young-ryeol, chief prosecutor of the Seoul Central District Prosecutors' Office said.

In a televised news conference, Lee said that, based on the evidence, "the president was involved as a conspirator in a considerable part of the criminal activities by suspects Choi Soon-sil, Ahn Jong-beom and Jung Ho-sung."

Choi is a close confidante of Park's and Ahn and Jung are presidential aides who were also formally charged.

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"We will continue to investigate the president," Lee told reporters, declining to give further details.

Choi is accused of using her ties with the president to coerce local firms to donate millions of dollars to non-profit foundations that Choi then used for personal gain.

The saga has prompted weeks of protests in Seoul.

Lee said abusing authority, attempted fraud and attempted coercion were among the charges against Choi and Park's former aides.

Al Jazeera's Wayne Hay, reporting from Seoul, said the majority of South Koreans had been expecting such an announcement.

"However, what people were also waiting to hear was how much involvement the president had in these charges. If you read between the lines, the prosecutor seems to be saying that if it was not for the immunity, Park might have been charged," Hay said. 

Park's approval ratings have plunged and if prosecutors go ahead with their plans, she will be the first sitting South Korean president to be questioned in a criminal case.

Fourth round of protests

The president has defied calls to step down, but her lawyer recently said that she would cooperate with public prosecutors.

Separately, opposition parties used their parliamentary majority to pass a law on Saturday that would allow for a special prosecutor to investigate the scandal.

READ MORE: Park Geun-hye: Scandal is all my fault and mistake

Tens of thousands of protesters gathered in the capital Seoul on the same day for the fourth in a weekly series of demonstrations aimed at forcing Park to resign.

Opposition parties have yet to seriously push for Park's impeachment because they fear triggering a backlash from conservative voters, which could hurt them in next year's presidential election.

Park's term lasts until February 24, 2018. If she steps down before the presidential vote on December 20, 2017, an election must be held within 60 days.

Source: Al Jazeera And Agencies