By Ju-min Park

SEOUL (Reuters) - South Korean prosecutors have detained a former aide to President Park Geun-hye, a prosecution official said on Thursday, the second person to be held in an influence peddling scandal that has rocked the country's presidency.

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 adviser 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 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.

CALLS FOR PRESIDENTIAL INVESTIGATION

Members of opposition parties have called for prosecutors to investigate Park, which would be a first for a sitting South Korean president.

Her nominee to be prime minister, Kim Byong-joon, said on Thursday he believes the president could theoretically be subject to a probe.

"My position is an investigation is possible. But since (she) is the head of state, its process and method requires circumspection," Kim told a press conference.

Park is due to give an address on Friday morning, Yonhap News Agency said, adding that she may allow prosecutors to investigate herself. The presidential office was not immediately available for comment.

Park, 64, 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.

Despite numerous scandals over the years, no South Korean president has ever resigned or been successfully impeached. If Park resigned, an election would be held in 60 days, with the winner serving five years, making for a high-stakes race for which neither of the main parties has prepared.

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 center of the scandal, and are seeking to charge her with abuse of power and attempted fraud, court and prosecution officials said. Choi was placed under temporary emergency detention late on Monday.

OLD FRIEND

The Seoul Central Court granted the warrant to arrest Choi late on Thursday, a court official said.

A formal arrest warrant allows her to be held for up to 20 days.

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.

Choi's late father Choi Tae-min headed a now-defunct religious sect and was close to Park during and after the presidency of her father, Park Chung-hee, who was assassinated in 1979. A 2007 U.S. diplomatic cable described the senior Choi as the "Korean Rasputin", an allusion to his perceived influence over Park Geun-hye.

Choi Soon-sil has been portrayed in Korean media as having inherited her father's influence over Park.

(Reporting by Ju-min Park; Editing by Tony Munroe, Raju Gopalakrishnan and Mike Collett-White)