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Some two million people are expected to join nationwide protests in South Korea to demand the resignation of President Park Geun-hye, organisers say.
They estimate 1.5 million will gather in Seoul and 500,000 in other regions - in what would be largest rallies since the demonstrations began five week ago.
About 25,000 police are being deployed in the capital, local media report.
Ms Park is accused of allowing her friend, Choi Soon-sil, to manipulate power from behind the scenes.
The president has apologised twice on national television, but has so far resisted calls to resign.
The fallout from the scandal shows no sign of abating, with South Korea witnessing the largest protests since pro-democracy demonstrations of the 1980s.
On Saturday, members of the Korean Peasants League, the country's largest farmers group, are expected to join the protest in Seoul.
The farmers had wanted to bring 1,000 tractors and other machinery near the government quarter - but were banned from doing so by a court order.
Local media reported scuffles on Friday, as police blocked groups of farmers trying to enter the capital in their vehicles.
But they were allowed to continue on foot to join Saturday's rallies.
The scandal is centred on the close relationship between Ms Park and Ms Choi. Photo: AP
The organisers said as many as 500,000 people attended a peaceful candlelit rally in the capital last weekend, bringing streets to a standstill.
Police put the figure far lower.
The big question - by BBC's Stephen Evans in Seoul
Convoys of farmers have been prevented from taking their tractors into Seoul - but the streets have been packed with people, chanting that President Park Geun-hye should step down.
Apart from the corruption allegations, she has become the focus of discontent over the economy. Farmers, for example, spilled sacks of rice on the road in protest at low rice prices.
Courts have permitted demonstrations up to 200 metres from the presidential palace but only before nightfall.
Tens of thousands of police are deployed, with the full panoply of anti-riot gear.
The question is whether the more militant elements will disperse or try to get to the presidential palace.
Ms Park, whose approval rating has dropped to 5%, apologised earlier this month for putting "too much faith in a personal relationship", and has pledged to co-operate in an official investigation into the scandal.
South Korea's constitution does not allow a sitting president to be prosecuted, and Ms Park has 15 months left in her term.
But now that prosecutors have directly linked her to the scandal, it is possible she could be impeached for breaking the law.
Prosecutors are expected to bring charges against Ms Choi, along with two former presidential aides. She was arrested earlier this month.
Ms Choi is accused of trying to extort huge sums of money from South Korean companies, and suspected of using her friendship with Ms Park to solicit business donations for a non-profit fund she controlled.
(BBC)
