Protesters oppose plans including changes to labour laws and greater controls over history textbooks. Left-wing critics say this will lead to a whitewashing of past South Korean dictatorships.
Park Geun-hye, the country`s first female president, was elected two years ago.
She is also pushing through controversial moves to make labour markets more flexible by giving employers more leeway in dismissing workers. About 60,000 people took part in the first rally, with saw violent clashes between protesters and police.
A 69-year-old fell into a coma after being hit with water cannon. Correspondents say Saturday`s rally is likely to include a march on the hospital where he is being treated.
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