Organisers claimed a turnout of 1.5 million for the candlelight rally in the South Korean capital, while police put the number at 220,000.
It was the latest in a series of massive anti-Park demonstrations and came just hours after opposition parties filed an impeachment motion that will be put to a vote by lawmakers yesterday.
Whether the motion is adopted or not, Park is firmly on course to become the first democratically-elected South Korean president not to complete a full, five-year term.
Along with the now-normal slogans for Park to step down, there were growing calls at today's rally for her to face criminal charges, arrest and imprisonment.
Dozens of life-size cardboard cut-outs of the president showed her wearing jail uniform and bound by ropes.
While the protesters want her out immediately, the political establishment is struggling to find a similar unity of purpose.
The impeachment motion introduced in the early hours of today morning carried 171 signatures -- accounting for every legislator from the three opposition parties and independents.
Just a week ago, the backing of enough Saenuri rebels seemed assured, but a rather confused resignation offer by Park on Tuesday strengthened the hand of her loyalists who insist she be allowed to step down voluntarily.
The party has since proposed she resign in April -- a timeline it justifies as more conducive to a calm and steady preparation for an early presidential election.
Observers say the Saenuri rebels are likely to fall in line with the proposal and vote against the motion on Friday.
"I no longer believe a word the president or her party says," said bank employee Kim Hak-Won who was marching today with his teenage daughter.
"How can we tell our children to respect the law when our own president refuses to do so?" Kim said.
And there was widespread anger with the Saenuri party over what were seen as its efforts to block the impeachment process.
"I am so full of rage right now, I could set fire to the party headquarters," said 30-year-old office worker Park Sung-Jin.
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