Up to 1.3 million protesters braved sleet and freezing temperatures in Seoul on Saturday to demand President Park Geun-Hye resign over a corruption scandal or face impeachment, organisers said.
Participants raised candles, sung and danced while chanting "Arrest Park Geun-Hye" and "Throw Park into jail", with cries from the main rally site reportedly reaching the presidential Blue House some 1.5 kilometers (0.9 miles) away.
The figure offered by organisers would make this the largest of a series of huge weekly protests that began a month ago in the South Korean capital, after an influence-peddling scandal engulfed the president.
Police put the turnout at 260,000.
At 8 pm (1100 GMT) demonstrators put out their candles, only to relight them a minute later as a warning that their protests would not burn out until Park left office.
"I don't think Park would step down voluntarily, but we need to raise our voice as much as possible to encourage parliament to push through with its move to impeach her," Lee Seung-Cheol, a 23-year-old student, told AFP.
The largely peaceful rallies - which have been attended by parents and their children, university students and Buddhist monks - are among the biggest seen in South Korea since the pro-democracy protests of the 1980s.
Park has issued public apologies over the scandal involving her long-time confidante Choi Soon-Sil, who has been arrested for fraud and abuse of power, but has defied repeated calls to resign.
Choi is also accused of interfering in government affairs, despite holding no official position.
The 60-year-old allegedly leveraged her relationship with Park to coerce donations from conglomerates, including SK, Lotte and Samsung, to non-profit foundations which she set up and used for personal gain.
Park has promised to submit herself to an expanding probe by prosecutors, as well as a separate investigation by an independent special prosecutor to be appointed by parliament.
Nevertheless her approval ratings have plunged to a record low for a sitting president as top advisers and some of South Korea's most powerful companies are caught up in the ever-widening scandal.
The headquarters of SK, Lotte and Samsung were raided by state prosecutors this week along with the offices of the finance ministry and state pension fund.
A parliamentary vote to impeach Park could take place as early as next week as a growing number of ruling party politicians back the opposition-led campaign to oust the president.
A poll this week indicates that nine out of 10 South Koreans want Park kicked out of office.
"I came here because I wanted to show my children that it's the people who own this country, not those in power," Shim Kyu-Il, a 47-year-old company employee, told AFP.
There was a festive mood among protesters, with many wearing raincoats and clutching umbrellas to protect themselves from the cold and wet weather.
Top singers joined the protest, turning the event into something like a giant rock concert interspersed with chanted slogans and mass dances.
Food, placards and leaflets were handed out, while street vendors sold candles and chairs.
Buddhist monks wearing grey robes recited a sutra while other protestors simulated Park, Choi and Samsung scion Lee Jae-Yong being led into prison.
Trucks carried loudspeakers blaring "Park get out now".
Yang Duk-Joon, 53, said he and other farmers had taken a bus from the southern provincial city of Muan to join the protest.
"We're here to oust Park who ruined this country," he said, adding the rice price had fallen 40 percent this year compared with a year earlier.
If parliament passes the impeachment motion, Park would be suspended from official duties and replaced by the prime minister. The Constitutional Court would need to approve the impeachment.
"Even though the Constitutional Court is deemed conservative, they would be unable to defy the people's wish to oust Park", Kang Won-Taek, a political science professor of the Seoul National University, told AFP.