Tens of thousands of people, many in Santa Claus outfits, marched through the streets of Seoul Saturday calling for the immediate ouster of impeached President Park Geun-Hye.
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.
Police figures were unavailable but organisers estimated the crowd at more than 550,000. Protesters walked in three columns towards the presidential Blue House, the prime minister's office and the Constitutional Court.
"Arrest Park immediately", they chanted, while also urging the Constitutional Court to approve the impeachment.
Despite sub-zero temperatures, protesters waved banners and balloons, and sang along to Christmas songs with new lyrics heaping ridicule on Park and calling for her immediate removal.
"This is a special Chrismas eve as it gives me a chance to show my children what democracy is all about", Yoon Ki-Seung told AFP as his son and daughter held banners.
Some 300 young people wearing Santa Claus outfits were seen handing out books and Christmas cards to children who accompanied their parents to the demonstration.
"Gifts to children and handcuffs to Park", they chanted.
Artists drew a large baby Jesus holding a candle with the slogan "Oust Park in the name of the people" on the pavement.
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.
The demonstration came as investigators were expanding a probe into the scandal to determine whether Park and Choi took bribes from conglomerates such as Samsung in addition to soliciting "donations" to the two foundations.
Samsung has been a main target of the investigation. It allegedly bribed Choi to win state approval for a controversial merger of two of its units in order to bolster its founding family's control over the management.
Prosecutors are also investigating new allegations that Choi 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 Saturday 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.
"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.