With the verdict, the ousted and jailed Park may face harsher legal punishment in a massive bribery scandal involving her longtime friend, Choi Soon-sil, and Samsung Group's leader Lee Jae-yong.
South Korea's highest court sent Park's case back to the Seoul High Court, ordering it to handle her bribery charge separate from other ones.
Last year, the appeals court sentenced her to 25 years in prison after convicting her of bribery, extortion, abuse of power and other charges all together.
It's likely to lead to a heavier penalty, observers said, as sentences for bribery and other charges will be aggregated purely and simply.
A sense of regret and disappointment was palpable among those gathered in front of the Supreme Court building in southern Seoul.
Police estimated the number of them at around 1,500.
Holding pickets and national flags, they were demanding the release of Park, claiming she's innocent.
Joining the street rally, Rep. Cho Won-jin, co-head of the Our Republican Party, characterized the verdict as a "political ruling" by a "hunting dog of the leftist, dictatorial Moon Jae-in regime."
On the other hand, liberal civic groups including the Korean Confederation of Trade Unions welcomed the court's decision on Lee, vice chairman of Samsung Electronics.
The court ordered a retrial for him as well, voiding the lower court's ruling to suspend Lee's 2 1/2-year prison sentence.
It concluded that Lee had paid far more kickbacks, which raises the possibility that he will be put behind bars.
Park Seok-un, head of Korea Alliance for Progressive Movement, said Lee deserves it.
The unchangeable truth is that Lee bribed Park and Choi, he said.
"The arrest of Lee is unavoidable with the ruling this time," he added. "The government and Samsung should deprive him of management rights." (Yonhap)