According to the organisers of the demonstration, some 1.5 million will gather in the capital city of Seoul and 500,000 in other regions demanding Geun-hye's resignation.
The police estimate of the turnout was much lower, at 260,000.
Mass protests against Park over the past four weekends have been the biggest public demonstrations seen in South Korea since the 1980s. "I would like to say that this is absolutely not true", she said. "We will not make the same mistake again".
People handed out food, placards and leaflets to the festive gathering. "And many Koreans call Park as "Choi's puppet" or Choi as Park's 'princess maker'". The pressure to establish a more sustainable model of governance is huge, not least because South Korea finds itself at a strategic crossroads on the global stage.
Park, the country's first female president has been accused of leaking official state documents to Choi.
"It's much more than simply, 'Oh she knows this person, ' it's deeply intertwined, nearly like they're Rasputin and Park Geun-hye is just a puppet", David Kang, director of the Korean Studies Institute at the University of Southern California, was quoted as saying.
Meantime, opposition parties are canvassing for support to impeach her. The National Assembly is expected to vote on an impeachment bill by December 9.
South Korean students wearing masks, holding candles.
Gallup Korea said Saenuri's approval rating also reached a record-low of 12 percent by dropping 3 percentage points on-week, falling behind the minor People's Party that posted 16 percent. She hasn't shown any signs of planning to step down.
Ms. Park wants to deploy a sophisticated US missile system next year to defend against North Korea's advancing nuclear-weapons program.
Even if she survives the tumult, Ms. Park's diminished political authority presents risks for the USA and an early foreign-policy challenge for President-elect Donald Trump.
Since the controversy, Park has issued a public apology and has promised to allow an interview with the prosecution team. "Come out and surrender!" they shouted. One group of demonstrators was stopped just 200 yards from the presidential compound.
Massive protests have broken out since late October, and the latest marked the fifth consecutive week of such an enormous demonstration. Rock musicians performed on the main thoroughfare, replacing the lyrics of popular songs with phrases that criticized and ridiculed Park.
Trucks carried loudspeakers blaring "Park get out now".
On smaller streets, citizens spoke to the crowd from the back of trucks.
Police said about 270,000 people turned out on Saturday, making it the largest anti-Park rally yet.
Buddhist monks wearing grey robes recited a sutra while other protestors simulated Park, Choi and Samsung scion Lee Jae-Yong being led into prison.
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.
They said Park must distance herself from state affairs, including diplomacy and security affairs.