Officials on Wednesday raided the offices of the Samsung Group, over an alleged link to an influence-peddling scandal that has implicated the president.

Prosecutors were also said to have raided the National Pension Service (NPS), the country's largest pension fund - and the third largest in the world.

Park and Choi are being investigated for allegedly pressuring major conglomerates, including Samsung, to donate to dubious non-profit organizations set up for personal gain.

Choi has been indicted on a string of charges, including the abuse of authority and attempted fraud. A former presidential aide, An Chong-bum, was also indicted.

Park - who ran on a "no corruption" ticket - is suspected of allowing her friend to use the presidential office as leverage with the businesses. She is also accused of leaking government documents to Choi, who has no security clearance or official position. 

Prosecutors are looking into an allegation that Samsung might have given 2.8 million euros ($3.1 million) to Park's friend, Choi Soon-Sil, in order to finance Choi's daughter's equestrian training. Choi's daughter was previously a member of the South Korean national equestrian team and trained in Germany.

'A collusive role'

The country's largest opposition party announced on Monday it would take steps to start impeachment proceedings against Park, after prosecutors said it was likely she played a role in the case. 

"The president played a collusive role in a considerable portion of the criminal activities involving the (three) people," said Seoul prosecutor Lee Young-Ryeol, who is leading the investigation.

Prosecutors have said they cannot indict Park currently, given that the country's constitution guarantees presidential immunity "except in cases of insurrection or treason."

Pressure has also been piled on the president, with weekly protests against her drawing hundreds of thousands of people nationwide. 

The NPS has been forced to defend a decision last year to approve the merger of a Samsung subsidiary, Samsung C&T Corp, with the Korean-based textiles and fashion business Cheil Industries. The NPS is a major shareholder in both firms, and the merger is believed to have come at the expense of minority shareholders.

 rc/bw (Reuters, AP, AFP, dpa)