Hundreds of thousands of South Koreans have held rival protests in Seoul over the impeachment of President Park Geun-hye on the fourth anniversary of her swearing into office.

About 17,000 riot police were mobilised on Saturday, setting up barricades with their buses at the centre of the capital to separate Park's supporters from those opposing her.

Anti-Park protesters, carrying yellow balloons 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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A third group passed by the headquarters of the powerful, family-run business groups - known as chaebols - including SK, Lotte and Hanwha.

Park was impeached by parliament in December over a corruption scandal that has since brought millions of people on to the streets in mass protests.

The de-facto head of South Korea's giant Samsung Group Lee Jae-yong was arrested last week, a first among business tycoons embroiled in the scandal surrounding the president and her confidante, Choi Soon-sil.

"Arrest other tycoons," the protesters chanted.

The chaebols directed millions of dollars into two foundations controlled by Choi.

They all denied providing funds in return for favours, but suggested that they regularly came under pressure from high-level political circles.

Park has seen her approval ratings fall from 67 percent to 5 percent as the country has been rocked by the scandal.

The Constitutional Court will on Monday hold its final hearing on Park's impeachment. The verdict is expected to be handed down before March 13.

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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.

Police stopped issuing their own estimates and an AFP journalist on the scene put both numbers around 100,000 each or less.