Almost one million protesters have gathered in the South Korean capital, Seoul, to demand the resignation of President Park Geun-hye.
Large numbers of police officers are being deployed to prevent demonstrators reaching the presidential palace.
Ms Park is accused of allowing her friend, Choi Soon-sil, to access government documents without clearance.
The president, whose approval ratings have plunged over the scandal, has said she is “heartbroken”.
Organisers said one million had turned out to protest on Saturday, while police put the number at 260,000.
It is the latest in a series of mass rallies against her over the issue.
Ms Choi is accused of trying to extort huge sums of money from South Korean companies and is under arrest on charges of fraud and abuse of power.
She was detained last week on suspicion of using her friendship with Ms Park to solicit business donations for a non-profit fund she controlled.
The focus of the protest is President Park Geun-hye, whose presidential compound (known as the Blue House because of its blue roof) is a short distance from the march and the main stage of the rally. If she is at home there, the noise will be inescapable.
Between 20,000 and 30,000 officers are on stand-by around the perimeter of the Blue House.
Many are in riot gear. Huge barricades have been erected, with water cannon in position.
At the moment, President Park does not face any criminal charges, but people close to her are being investigated for corruption.
Her close friend of 40 years’ standing has been charged with setting up two foundations and soliciting millions of dollars from big companies, including Samsung, on the strength of her closeness to the president.
Perhaps worrying for President Park is the fact that two officials from her office are also being investigated. She remains untouched but with the investigators circling. So are the crowds.
Ms Park has apologised following the revelations, saying she “put too much faith in a personal relationship and didn’t look carefully at what was happening.”
“Sad thoughts trouble my sleep at night. I realise that whatever I do, it will be difficult to mend the hearts of the people and then I feel a sense of shame.”
She said anyone found to have done wrong would be punished and that she was prepared to be investigated by prosecutors.
Prosecutors have also said they are expanding their official investigation.