The country's parliament impeached Park in December, and she was stripped of her presidential powers pending the court's judgment.
The ruling is expected to be televised live and takes place 11 a.m. (9 p.m. ET).
If the decision to impeach her is upheld, an election for her replacement must be held within 60 days.
Police issued the "Gabho" emergency order in Seoul ahead of Friday's announcement, the highest level possible.
All of police officers are on standby for expected protests, with 270 units being mobilized -- about 21,000 policemen in total.