Park is obliged to move out of the presidential palace, where she has been holed up for more than 90 days after the National Assembly voted for her impeachment in October.
By early afternoon, hundreds of protesters - many of them who had braved the biting Korean winter to take to the streets and demand Park's ouster late a year ago - were pouring into the square.
She was impeached by Parliament in December over a case involving her close friend Choi Soon-sil. The ani-Park protester in the second photo below holds a signs referencing the eight-member court's unanimous ruling.
Her actions betrayed the people's confidence.
"The removal of the claimee from office is overwhelmingly to the benefit of the protection of the constitution".
South Korea must hold a presidential election within 60 days and Moon, a former Democratic Party leader narrowly beaten by Park in 2012, has always been leading the polls. It was an unusually quick response from the rogue nation, which typically waits days to report global news, according to Reuters.
Acting South Korean President Hwang Kyo-ahn on Friday asked all parties concerned to respect the constitutional court's ruling on former President Park Geun-hye.
Watch Selena Gomez Groove to Nicki Minaj's 'No Frauds'
Nicki Minaj has been in the news every day since Remy Ma debuted her lethal diss track 'shETHER' against the Queens rapper. But if we know anything about the Terror Squad member and Big Pun descendant, it's that she is not one to back down.
Jay Y. Lee, the third-generation leader of the Samsung conglomerate, has been there since February 17, sleeping on a mattress on the floor.
"Park Geun-hye is an ignorant and uncouth dictator who squandered the taxpayer's precious money for her own complacency and pleasure", the piece read.
While the two had not seen eye to eye on every issue, together they made progress in one of the sticking points in bilateral relations springing from their countries' turbulent history.
Park, 65, becomes the country's first democratically elected leader to be ousted from office. She had refused to undergo questioning, saying as a sitting leader she was legally immune from prosecution.
Carrying signs and candles, the crowd cheered to speeches and swayed to music Friday night, while angry supporters of Park continued to scuffle with police in streets near the court.
"This is not just a victory for South Korea, this is a victory for real democracy", one commenter added.
The main opposition Democratic Party of Korea is home to several top candidates, including Mr Moon Jae In, 63, who lost to Ms Park by 3 percentage points in 2012.
"There was no reason for the impeachment", Jason Choi, 36, told CNN outside City Hall, where he stood draped in a South Korean flag. He followed months of political paralysis and turmoil over a corruption scandal that also landed the head of the Samsung conglomerate in jail and facing trial.