Samsung Group Vice Chairman Lee Jae-yong was questioned as part of a wider probe into corruption claims that could topple impeached South Korean President Park Geun-hye.
The latest protest drew hundreds of thousands of demonstrators, with 1,000 people flying in from the southern resort island of Jeju.
Local media accused the country's president of being a puppet of a Shamanic cult, and that has angered many people.
President Park Geun-Hye has been rocked by allegations that an old friend used her influence to meddle in state affairs and gain personally.
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