President Park Geun-hye could become the country's first elected leader to be expelled from office.
South Korea's Constitutional Court on Thursday began the hearing on a parliamentary impeachment vote against President Park Geun-hye over an influence-peddling scandal.
The impeachment motion was passed on December 9 by a wider-than-expected margin, with the approval of 234 lawmakers from the 300-seat parliament, setting the stage for her to become the country's first elected leader to be forced from office.
Park is accused of colluding with a friend to pressure big businesses into donating to two foundations set up to back her policy initiatives.
She denied any wrongdoing but apologised for carelessness in her ties with her friend, Choi Soon-sil, who is facing her own trial.
Park and Choi were not in the courthouse on Thursday as judges decided to admit investigation documents from prosecutors.
Park's lawyers earlier said that the vote to impeach Park had no legal basis and the court should throw the case out.
Park is serving a single five-year term which is set to end in February 2018.
She now has presidential immunity, but risks facing prosecution after leaving office.