Tens of thousands of South Koreans have protested in the capital, Seoul, to demand the resignation of President Park Geun-hye over a corruption row.
A long-time friend, the daughter of a cult leader, is accused of undue influence over the president.
Choi Soon-sil is alleged to have pushed businesses to donate millions of dollars to foundations she controlled, helped choose presidential aides, and even picked the president's clothes.
Ms Park apologised on TV on Friday.
Ms Park was close to tears as she addressed the nation, and said the scandal involving her confidante Ms Choi was "all my fault". The president admitted she had let Ms Choi edit her speeches.
Ms Choi was arrested on Thursday and charged with fraud and abuse of power.
Masses of protesters gathered in Gwanghwamun square in central Seoul on Saturday, chanting, singing and holding banners reading "Park Geun-hye out" and "Treason by a secret government".
Police estimated the size of the crowd at about 45,000, although protest organisers put the figure closer to 200,000.
Some 20,000 police officers were deployed, blocking off some routes with buses and trucks.
Demonstrator Choi Kyung-ha told Associated Press: "I came out today because this is not the country I want to pass on to my children. My kids have asked me who Choi Soon-sil was and whether she's the real president, and I couldn't provide an answer."
Smaller protests were held in other cities, including the southern city of Gwangju, where 3,000 gathered.
The president has denied media speculation that she took part in "shamanist rituals" at the Blue House, and promised to accept an investigation into her actions.
The affair has left Ms Park with an approval rating of just 5%, the lowest ever for a sitting South Korean president.
The opposition has demanded that she either resign, or accept a prime minister chosen by parliament as her number two.