Park was impeached by parliament in December over a corruption scandal that has brought millions of people onto the streets in mass protests.
Around 17,000 riot police were mobilised, setting up barricades with their buses at the centre of the capital to separate Park's supporters from those opposing her.
Anti-Park protestors, carrying yellow ballons and waving banners, marched in three directions.
One column proceeded towards the presidential Blue House as another group headed to the Constitutional Court, which has to decide whether to endorse or reject Park's impeachment.
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The de-facto head of South Korea's giant Samsung Group Lee Jae-Yong was arrested last week, a first among business tycoons engulfed in the scandal surrounding the president and her close friend Choi Soon-Sil.
"Arrest other tycoons," the protestors chanted.
The chaebols funnelled millions of dollars into two dubious foundations controlled by Choi.
They all denied providing funds in return for favours, but suggested they regularly came under pressure from high-level political circles.
Park has seen her approval ratings plunge from 67 per cent to five per cent as the country was rocked by the high-profile scandal.
If the court upholds the impeachment, Park will be removed from her post immediately and a presidential election must be held within 60 days.
Not too far from the anti-Park protest, the president's supporters, mostly senior citizens, waved national flags and chanted slogans urging the court to nullify the impeachment. Otherwise it would face a "bloody" civil resistance, they said.
Anti-Park protest organisers claimed a one million turnout and pro-Park supporters said they had attracted three million.