Police said about 260,000 people turned out for the latest mass rally against Park, whose presidency has been shaken by suspicion that she let a shadowy longtime confidante manipulate power from behind the scenes.
Protest organisers estimated the crowd at 1 million.
Waving banners and signs, a sea of demonstrators jammed streets stretching about a kilometre from City Hall to a large square in front of an old palace gate for several hours, roaring and applauding to speeches calling for Park's ouster.
Bae Dong-san, a 45-year-old man, said Park's government has "worsened the living conditions of workers, completely messed up state governance and monopolised state affairs with her secret inner circle."
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"It feels much better to shout together with many other people," he said.
Despite rising public anger, opposition parties have yet to seriously push for Park's resignation or impeachment over fears of triggering a backlash from conservative voters and negatively impacting next year's presidential election.
The protest on Saturday was the largest in the capital since June 10, 2008, when police said 80,000 people took part in a candlelight vigil denouncing the government's decision to resume US beef imports amid mad cow fears.
Organisers estimated that crowd at 700,000. In the summer of 1987, millions rallied in Seoul and other cities for weeks before the then-military government caved in to demands for free presidential elections.
Train and express bus tickets to Seoul were difficult to get from some areas last evening and this morning, with the protest reportedly drawing tens of thousands of people from other cities.
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