Park was impeached by parliament last December over a major influence-peddling scandal that prompted millions to take to the street for months to call for her resignation.
The case has also engulfed South Korea's biggest business, with the heir to the Samsung empire and four other top executives indicted Tuesday on multiple charges including bribery and embezzlement.
Park - currently holed up in the presidential palace with her power suspended - expressed "belated regret", as the Constitutional Court wrapped up its hearings on Monday, with a decision expected in March.
The case has bitterly divided Park's supporters and detractors, who have regularly taken to the streets.
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Tens of thousands of anti-Park protesters rallied in Seoul today, some waving banners of the president in prison uniform and chanting in unison "Constitutional Court, impeach Park immediately!"
Several blocks away, another much smaller demonstration demanded the court reject the impeachment against the "poor innocent president".
While the court's deliberations could technically take up to six months, local media reports predict the judgement could come as early as next week.
The impeachment motion needs approvals from at least six judges to take effect, after which a presidential election will be held within 60 days.
If it is rejected, Park could return to work and stay until the end of her term in February 2018.
Choi is on trial for coercion and abuse of power.
The two women became close through Choi's father, a shady religious figure who was Park's mentor for years.
The 65-year-old president is accused of colluding with her friend to extract money from local firms as well as letting her handle a wide range of state affairs including nominations to senior posts, despite having no official title or security clearance.
A number of former presidential advisors and cabinet members have been arrested or charged for their involvements in the scandal, including allegedly leaking secret state documents to Choi at Park's order.