Parliament voted to impeach Park earlier this month over a corruption scandal in which she allegedly colluded with her friend.
Parliament voted to impeach Park earlier this month over a corruption scandal in which she allegedly colluded with her friend, Choi Soon-Sil, to strong-arm donations from large conglomerates to two dubious foundations.
Park is also accused of ordering aides to leak confidential state documents to Choi, who has no official title or security clearance, and allowing her to meddle in state affairs, including the appointment of top officials.
Massive demonstrations have been taking place in Seoul and other cities every Saturday for the past two months, with protesters calling for Park's immediate departure from office.
But 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 Constitutional Court, which is due to rule on the case within 180 days, arrives at a decision.
Organisers said Saturday's protest would see demonstrators sing along to Christmas tunes with new lyrics calling for Park's immediate removal.
"Today's demonstration will be a mix of Christmas-eve celebrations and protests," Ahn Jin-Geol told AFP.
The demonstration comes as investigators prepared to question Choi in connection with new allegations that she sent dubious assets worth hundreds of millions of dollars overseas.
Choi, who has been in custody since October, was brought to a special prosecutors' office in a prison bus.
Television footage showed the 60-year-old handcuffed and wearing an ivory prison outfit with a serial number on the chest as she was taken off the bus and led into the court building.
Choi, dubbed South Korea's "female Rasputin" for the influence she wielded over Park, faces trial on charges of embezzlement and abuse of power -- largely related to huge "donations" made by companies to two foundations she controlled and allegedly plundered.
"We will question her to confirm her earlier statements and investigate other allegations," a spokesman for the special prosecutor told reporters.
Choi's appearance came on the heels of the first hearing into Park's impeachment earlier this week.
If the justices confirm impeachment, Park will be permanently removed and elections must be held within 60 days.