More than 70 percent of the country wanted Park removed from office but the decision has sown deep divisions. Authorities warned the military to be wary of North Korean attempts to capitalize on the situation.
South Korean authorities deployed more than 20,000 police to the streets of Seoul to maintain order on Friday following the ousting of President Park Geun-hye.
Two people died in the capital city in protests against the Constitutional Court's decision to remove President Park Geun-hye from office.
The eight-judge panel had upheld a Parliamentary vote to impeach Park for a graft scandal, opening the way for criminal proceedings. Park's "acts of violating the constitution and law are a betrayal of the public trust," acting Chief Justice Lee Jung-mi said. "The benefits of protecting the constitution that can be gained by dismissing the defendant are overwhelmingly large."
Prosecutors had already named her a criminal suspect, making her South Korea's first democratically elected leader to be removed from office since democracy replaced dictatorship in the late 1980s. The president's confidante Choi Soon-sil was accused of extorting money from conglomerates such as Samsung.
The decision has revealed deep divisions between the majority who supported Park's impeachment and her staunch supporters.
Several thousand Park supporters clashed with police on the streets as officials called for calm.
One man in his 70s died from head wounds during heated protests in the nation's capital, according to a hospital statement. No details were released of the other death.
Park's lawyer, Seo Seok-gu, who previously compared her impeachment to the crucifixion of Jesus Christ, called the verdict a "tragic decision" made under popular pressure and questioned the fairness of what he called a "kangaroo court."
Polls showed more than 70 percent of South Koreans wanted the court to approve Park's impeachment, after hundreds of thousands of protesters spent weeks calling for her impeachment.
North Korean provocations
South Korea's defense minister ordered the military to watch for possible North Korean provocations attempting to exploit "unstable situations at home and abroad."
During a Friday video conference with military commanders, Han Min Koo warned North Korea could make "strategic or operational" provocations at any time. In recent weeks North Korea test-fired ballistic missiles.
North Korea seized on the court ruling saying Park would now come under investigation as a "common criminal."
The decision deepened South Korea's political and security uncertainty amid muscle-flexing from North Korea, a reported economic retaliation from China following Seoul's cooperation with the US on an anti-missile system, and questions in Seoul about the new Trump administration's commitment to the countries' decades-long security alliance.
In a satirical display of celebration on Friday many South Koreans ate chicken, a reference to the former leader's derogatory nickname, "Chicken Geun-hye." The name is both a play on words - her family name rhymes with chicken in Korean - and a jibe against her percieved lack of intellect and stilted public speaking.
A restaurant in the rural city of Jeonju offered chicken burgers at half price to celebrate the court ruling.
"We prepared twice as many chicken burgers as usual and, wow, they've almost all gone," Yu Yeung-sang, owner of Eddis Kitchen, told news agency Reuters.
Japan's Foreign Minister Fumio Kishida told reporters the court was a case for internal politics, but said he hoped he could work with the new leadership to resolve an impasse over Japan's WWII exploitation of so-called "comfort women." Japan recalled its ambassador two months ago over the squabble.
The United States said the decision was a domestic issue that didn't affect its strong alliance with the country.
State Department spokesman Mark Toner said the two nations' alliance "will continue to be a linchpin of regional stability and security."
Toner said the US would continue to work with the acting president, Prime Minister Hwang Kyo-ahn, and looked forward to whomever South Koreans choose in a presidential election to be held within two months.
Park was toppled by a corruption and influence-peddling scandal that centered on her confidante Choi Soon-Sil. Choi herself is on trial for using her ties to Park to bully Korean conglomerates to "donate" about 80 billion won (66 million euros, US$70 million) to non-profit foundations Choi controlled.
Park was named as Choi's accomplice and was accused of letting her handle a wide range of state affairs, including senior nominations.
Prosecutors arrested and indicted many high-profile figures over the scandal, including administration officials and Samsung heir Lee Jae-yong. Park had avoided investigation due to her immunity as a sitting President but now faces prosecution.
Lee Jae-yong, the de-facto Samsung boss, allegedly gave or promised 43.3 billion won ($38 million) to four entities controlled by Choi in a bid to obtain approval for a contentious merger of two Samsung companies in 2015.
South Korea needs to hold an election within two months to choose Park's successor.
Acting leader, Prime Minister Hwang Kyo-ahn said he would work with the cabinet to stabilize the country and hold elections.
"I respect the constitutional court's decision ... The cabinet should carry out state affairs in a stable way and manage social order to prevent internal conflict from intensifying."
Local media reported he may run for president as a conservative candidate.
Liberal Moon Jae-in, who lost to Park in the 2012 election, had a comfortable lead in recent polls.
aw/jm (AFP, AP, Reuters, dpa)