SEOUL (Reuters) - A high-flying prosecutor, who was demoted under impeached former president Park Geun-hye, is overseeing the case against her as Park made her first appearance in court on Tuesday to face criminal charges over the corruption scandal that ousted her.

Yoon Seok-youl, chief prosecutor of the Seoul Central District Prosecutors' Office, attends an event at the Supreme Prosecutor's office in Seoul, South Korea, May 22, 2017. Picture taken on May 22, 2017. Min Kyung-seok/News1 via REUTERS

Yoon Seok-youl, 57, was demoted for defying his boss’s order to back off a 2013 investigation into Seoul’s spy agency over its suspected efforts to illegally support Park Geun-hye’s 2012 presidential bid. At the time, leading a special investigation team, Yoon pressed on with the probe anyway, arresting spy agency officials and raiding their offices.

Yoon was subsequently suspended from the investigation and demoted to relatively trivial posts outside of Seoul.

Handpicked by new liberal President Moon Jae-in last week to head the Seoul Central District Prosecutors’ Office, Yoon now oversees the case against Park, who was arrested in March over charges she took bribes from big business leaders.

On Tuesday, Park denied any wrongdoing and pleaded not guilty on the first day of the arguments. Moon, who took office two weeks ago, has promised to get tough on chaebol bosses who commit crimes.

Yoon was nicknamed the ‘Swordsman’ for prosecuting the country’s two most powerful businessmen - Samsung Group scion Jay Y. Lee is in jail while undergoing trial on charges of bribing Park, while Hyundai Motor Group Chairman MK Chung was given a suspended sentence and later pardoned in a 2006 corruption case.

Proving Park guilty would be a lengthy, uphill battle even for Yoon with hundreds of witnesses expected to be called, lawyers say.

COLLUDING WITH FRIEND

Prosecutors accuse Park of colluding with her friend Choi Soon-sil in taking bribes from corporate bosses, including about 29.8 billion won ($37 million) from Samsung, in exchange for business favors.

Park, her friend Choi and Samsung’s Lee have all denied any wrongdoing.

“So far in this case, the court of public opinion has been trying Park in a jumble of political and legal issues. But she’s been impeached, that’s over,” said lawyer Kang Shin-up.

“Now it’s a matter of criminal guilt or innocence, in which the judge has the initiative.”

Yoon did not appear at court on Tuesday. Spokespeople for the Seoul Central District Prosecutor’s Office could not be reached for comment.

In Park’s trial, the prosecution will seek to build evidence from material gathered over months of investigation, including thousands of pages of witness statements and some 39 notebooks prosecutors secured from one of Park’s former aides.

“Although prosecutors bear a considerable burden of proof, they have already gathered much material, so it will be a process of calling all the people involved and proving their case point by point,” said a lawyer who was a member of the special prosecutors’ team probing allegations against Park and corporate leaders. The lawyer declined to be identified.

Yoon returned to the public spotlight in late 2016 when he was asked to join the special prosecution team charged with investigating Park’s corruption scandal.

When asked whether he will “retaliate” against Park in return for his earlier demotion, Yoon said in December: “If a prosecutor retaliates with investigative rights, he is a gangster, not a prosecutor.”