Park Geun-hye faces charges including bribery, abusing state power and leaking state secrets.
In her first appearance in public since her arrest in March, Park Geun-hye arrived at court handcuffed in a prison van.
The maximum sentence for corruption in South Korea is life.
Park Geun-hye is accused of colluding with her friend, Choi Soon-sil, to extort money from some of South Korea’s biggest companies, including Samsung, in return for political favors.
Choi Soon-sil, who is also facing charges, sat alongside Park Geun-hye at the opening of the trial. She also denies wrongdoing.
As the trial opened in Seoul, Park Geun-hye’s lawyers said there was “no reason for President Park to force companies to donate money which she was unable to use for herself”.
She is facing a total of 18 charges, with the charge sheet running to about 120,000 pages, South Korean media report.
Prosecutors say she allowed Choi Soon-sil to use her presidential connections to pressure companies to give money to a foundation she established, earning them tens of millions of dollars.
In exchange, the companies would receive favorable treatment from the government, it is alleged.
Park Geun-hye is also accused of leaking state secrets by giving Choi Soon-sil access to her work – including asking her to edit her speeches – and of running a blacklist of media figures who were to be barred from receiving state support because they had been critical of her government.
The ousted president is the third South Korean leader on trial for corruption but the first democratically elected one.
The two previous cases involved former military dictators imprisoned on corruption in the 1990s.
Park Geun-hye’s hearing takes place in the very same court room as the previous trials.
Dozens of people have now been investigated or arrested in connection with the scandal. All have maintained their innocence.
Choi Soon-sil has been on trial since December, while her daughter, Chung Yoo-ra – alleged to have received help getting a university placement – was arrested in Denmark in January.
The acting head of Samsung Group, Lee Jae-Yong, and four other Samsung executive are on trial for allegedly giving bribes to Park Geun-hye and Choi Soon-sil in exchange for government support for a controversial merger.
Former health minister Moon Hyung-pyo and the chief investment officer of the National Pension Service (NPS) Hong Wan-seon are on trial for allegedly pressuring the NPS to accept the merger.
Last week, two plastic surgeons who treated the image-conscious former president – including one who injected Park Geun-hye with human placenta extracts – were convicted of lying about having treated her.
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