Prosecutors have said they are looking into allegations that the former senior adviser, An Chong-bum, and Choi Soon-sil, a long-time friend of the president, forced South Korean conglomerates to donate funds to non-profit foundations using their connections with the president.

An, who was a senior presidential advisor in policy coordination until he stepped down late last month amid the growing political crisis, was detained late on Wednesday. He earlier told reporters outside prosecution offices that he would take responsibility for his actions but declined to elaborate.

An's lawyers were not immediately available for comment early on Thursday. Prosecutors placed him under emergency detention, worried that he could destroy evidence, the prosecution official told Reuters.

Under South Korean law, prosecutors have 48 hours to seek an arrest warrant from a court.

On Wednesday, Park replaced her prime minister and finance minister, a reshuffle denounced by political opponents as a bid to divert attention from a crisis that has pushed her approval rating to an all-time low.

Members of opposition parties have called for prosecutors to investigate Park.

She has also faced calls from political opponents and a growing number of South Koreans to step down, although the main opposition parties have not raised the idea of launching impeachment proceedings.

Last weekend, Park accepted the resignations of eight of her top presidential aides, and on Thursday her office announced nominees for a new chief of staff and senior secretary for political affairs.

Prosecutors on Wednesday asked a court for a warrant to arrest Choi, Park's friend at the centre of the scandal, and are seeking to charge her with abuse of power and attempted fraud, court and prosecution officials said. Choi was detained late on Monday.

Choi, who has been a friend of Park's for decades but held no formal government role, is alleged to have used her proximity to the president to meddle in state affairs, and her lawyer has said he expects prosecutors to look into whether she inappropriately received classified documents.

Choi (60) told South Korea's Segye Ilbo newspaper last week that she received drafts of Park's speeches after Park's election victory but denied she had access to other official material, influenced state affairs or benefited financially.