Hundreds of thousands of protesters 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, BBC reported. President Park is accused of allowing her friend, Choi Soon-sil, to access government documents without clearance.
The president, whose approval ratings have plummeted over the scandal, has said she is "heartbroken".
Organizers 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.
Accused of trying to extort huge sums of money from South Korean companies, Choi is under arrest on charges of fraud and abuse of power. She was detained last week on suspicion of using her friendship with Park to solicit business donations for a non-profit fund she controlled.
Park has apologized 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 realize 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.
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.
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