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 kilometres 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 (local time) 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.
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.
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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.
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.
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.
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