Cameras flashed as South Korea's first female leader emerged from a prison bus, her inmate number, 503, attached to her blue jacket, and walked into Seoul central district court.
Choi, the daughter of a shadowy religious figure who was Park's mentor for years, is similarly accused of using her presidential ties to force top firms to "donate" almost $70 million to non-profit foundations which she then used for personal gain.
Park has been charged with extortion, bribery and abuse of power.
Her alleged co-conspirator, Choi Soon-sil, sat nearby.
Park's next court appearance is set for Thursday with up to three hearings planned each week until a verdict is reached within five months that could see her imprisoned for decades. Prosecutors also said they will cooperate with the court for a fair and speedy trial, and make sure that the procedural rights of those facing charges are guaranteed. We hope the Park-Choi trial offers priceless lessons for our presidents in the future.
The parliament voted to impeach Park in December, marking the second impeachment of an incumbent president in the country's history.
Choi is accused of using her presidential ties to force top firms to "donate" almost $70m to non-profit foundations which she then used for personal gain. She is also accused of illegally influencing appointments made by her government and Hana Bank and leaking confidential government information to Choi.
Park is accused of colluding with her close aide Choi Soon-sil and persuading major companies, including Samsung Group to provide financial assistance to two of the latter's foundations. The ousted president faces 18 charges, including receiving or demanding around 53 million USA dollars in bribes from conglomerates such as Samsung and Lotte, while Choi is also charged with receiving bribes. On the first day of the biggest trial in South Korea in years, two small gestures reflect the state of a friendship that toppled a president.
Park faces more than 10 years in jail if convicted. She also allegedly allowed her friend to manipulate state affairs from the shadows, despite having no title or security clearance.
1998: After years of avoiding the public eye, Park enters politics and wins a parliamentary seat amid public nostalgia for her father that erupted after South Korea was battered by the Asian financial crisis. "But those supporting this process, say while this is partly about [Park], it's important for South Korean democracy that this process goes ahead". Choi was seated between Park's lawyer and her lawyer, Lee Kyung-jae.
Park was handcuffed and wore a navy pantsuit, her hair held back in a haphazard bun and her face had little sign of make-up.
Lee has denied using the payments to win support for the 2015 deal, saying Samsung was just responding to Park's requests to support culture and sports.