Tens of thousands of people have gatehred in the South Korean capital of Seoul to demand the resignation of President Park Gun-hye.

Between 20,000 and 30,000 police officers are being deployed to prevent demonstrators reaching the presidential palace, estimating that there may be 100,000 people demonstrating.

Ms Park 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. She is currently not facing criminal charges.

The president, whose approval ratings have plummeted over the scandal, has said she is "heartbroken".

It is the latest in a series of mass rallies against her over the issue.

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.