Parliament voted to impeach Park Geun-Hye earlier this month over a corruption scandal.
Parliament voted to impeach Park Geun-Hye earlier this month over a corruption scandal in which she allegedly colluded with a friend to strong-arm donations from large conglomerates to two dubious foundations.
She is also accused of ordering aides to leak state documents to friend Choi Soon-Sil, who has no official title or security clearance, and allowing her to meddle in state affairs including the appointment of top officials.
The case is now being considered by the Constitutional Court which has 180 days to rule on the validity of the impeachment.
Justice Lee Jin-Sung rejected an apparent attempt by Park's lawyers to stall the hearing, instead launching into questions over where she was in the aftermath of the Sewol ferry disaster that left more than 300 people dead.
"I'm sure the president herself will know best what she did on the day of the Sewol ferry disaster," Lee said. "We request that she provide all details."
Park's response to the tragedy was one of the issues cited when parliament voted for her impeachment and Park faces growing pressure to explain what she was doing on the day.
Questions have been raised over Park's activities during a seven-hour period after she was initially informed of the sinking and before her first appearance at an official meeting to discuss the government's response.
Unconfirmed media reports have suggested a wide range of theories about Park's whereabouts, including a romantic liaison, participation in a shamanistic ritual, cosmetic surgery or, most recently, a 90-minute haircut.
Massive demonstrations have been taking place in Seoul and other cities every Saturday for the past two months, with protestors calling for Park's immediate departure from office.
The impeachment process was ignited and fuelled by public outrage at Park's behaviour, with the weekly mass protests demanding that politicians take a proactive role in removing her from the presidential Blue House.
The National Assembly has played its part, but the country now faces a lengthy period of uncertainty at a time of slowing economic growth and elevated military tensions with nuclear-armed North Korea.
Park, who has been suspended from her duties since the impeachment vote on December 9, has remained defiant, declaring she will "calmly" wait until the conservative-leaning Constitutional Court reaches a decision.
The impeachment has set off an early scramble among her potential successors, even before a final decision has been made to remove her from office.
South Korea's ruling conservative party is at the same time facing an imminent split over the issue, which could seriously undermine its election chances.
More than 30 Saenuri lawmakers who have been at odds with Park, including a few leading presidential hopefuls, declared Wednesday to leave the party to create a new reformist unit.
A split could complicate any potential presidential run by outgoing UN Secretary General Ban Ki-Moon, who had been widely expected to compete on the Saenuri ticket, leaving him without an obvious platform to launch and run his campaign.
Ban strongly hinted that he would jump into the presidential race when he met South Korean journalists in New York Tuesday, saying he was willing to devote himself to developing South Korea.
It remains unclear whether Ban will join the Saenuri Party or the emerging conservative party.
If the justices confirm impeachment, Park will be permanently removed and elections must be held within 60 days -- meaning a ballot could be held as early as late March.
At least six members of the nine-judge panel are needed to approve the motion, and Park is apparently pinning hopes on the court's conservative bent.
Choi, who is said to have had a "Rasputin-like" influence over the president, pleaded not guilty to all charges when her trial opened this week.