Tens of thousands of protesters have gathered in the South Korean capital, Seoul, to demand the resignation of President Park Gun-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 plummeted over the scandal, has said she is “heartbroken”.
Last week, protesters gathered in central Seoul chanting, singing and holding banners reading “Park Geun-hye out” and “Treason by a secret government”.
A friendship too far in Seoul?
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
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.
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.
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