South Korean President Yoon Suk Yeol was ousted on Friday by the Constitutional Court, ushering in an election after it upheld parliament's impeachment over his imposition of martial law that sparked the nation's worst political crisis in decades.

The unanimous ruling caps months of political turmoil that have overshadowed efforts to deal with the administration of U.S. President Donald Trump at a time of slowing growth in Asia's fourth-largest economy, Reuters reported.

The decision now sets in motion a race to elect the next president within 60 days as required by the constitution. Prime Minister Han Duck-soo will stay on as acting president until a new leader is sworn in.

Lee Jae-myung, the populist leader of the liberal Democratic Party who had lost to Yoon by a razor-thin margin in 2022, is a clear front-runner but faces legal challenges of his own under multiple trials for corruption. The conservatives have a wide-open field of candidates.

"The Constitutional Court's unanimous ruling has removed a major source of uncertainty," said Professor Leif-Eric Easley of Ewha University in Seoul, noting how the next government would have to navigate challenges including North Korea's military threats, China's diplomatic pressure and Trump's trade tariffs.

Acting Chief Justice Moon Hyung-bae said Yoon violated his duty as president with his December 3 martial law declaration, acting beyond his constitutional powers with actions that were "a serious challenge to democracy".

"(Yoon) committed a grave betrayal of the people's trust," Moon said, adding that the declaration of martial law created chaos in all areas of society, the economy and foreign policy.

Human Rights Watch called the ruling a win for the country's resilience, quest for human rights and democratic values.

Thousands of people at a rally calling for Yoon's ouster erupted into wild cheers on hearing the ruling, chanting "We won!"