SEOUL —The political showdown in South Korea opened Monday with the constitutional court ruling against impeaching the acting president, a soft-spoken economist and diplomat who catapulted to the top after the impeachment of the conservative president, Yoon Suk-yeol. 

The court, highly divided between conservatives and liberals, is still weighing whether to dismiss or accept the impeachment of Mr. Yoon. Tens of thousands of protesters, right and left, are massing on  central Seoul’s broad avenues. The leftist-dominated National Assembly passed a motion to dismiss Mr. Yoon in mid-December two weeks after voting down a martial law decree that he had to rescind within hours after issuing it.

The ruling against the impeachment of the acting president, Han Duck-soo, was accompanied by cheers and protests that should reach a crescendo with the decision on Mr.Yoon— and may go on long afterwards.  More than 10,000 policemen are guarding downtown streets, rerouting buses, and blocking wild-eyed demonstrators from colliding in violence as threatened by hotheads on both sides.

 The decision to reinstate Mr. Han as acting president showed the impatience of justices with leftists in the assembly. The eight judges were sharply divided, two totally on his side,  five for reinstating him with reservations and only one for his impeachment. The ruling may also provide a clue to the court’s thinking as the nation awaits the fate of Mr. Yoon. 

Many observers predict the outcome of the court’s protracted deliberations by Friday.  The country has been expecting a ruling on the president for weeks and may have to keep waiting while protests swell in noise and numbers.

A career bureaucrat with a doctorate in economics from Harvard, Mr. Han served as ambassador to Washington between 2009 and 2012 and was named prime minister by President Yoon after his election by a narrow margin in 2022. Mr. Han had served once before as prime minister under a liberal president, Roh Moo-hyun, and as an economics minister who helped craft the historic Korea-U.S.  Free Trade Agreement known as KORUS.

As shocked as he was to find himself the  acting president, Mr. Han was surprised again when the assembly impeached him for vetoing bills for investigating Mr. Yoon and his wife for corruption. He was also accused of failing to fill with leftists three vacancies on the nine-member constitutional court. 

Mr. Han’s immediate response to his exoneration by the court was to appeal for unity among a citizenry sharply divided along political, social and regional lines. “The majority of the people,” Yonhap News quoted him as saying “did not want the country to lean to the left or to the right.” Rather, “They simply wanted it to move upward and forward, rising and progressing,”

That message was largely lost among  those on both sides of the divide. While rightists listened to speakers blaring slogans and singing patriotic songs  from a huge sound truck, leftists marched down an intersecting avenue near the constitutional court.  Groups on both sides shouted outside the barricaded entrance to the court .

With the votes of six justices needed to approve Mr. Yoon’s impeachment and oust him from office, forcing a “snap election” for a new president in 60 days, leftists decried the reinstatement of Mr. Han as acting president. Earlier, they promised to introduce a motion for impeaching the economics minister, Choi Sang-mok, who had filled in as acting president after Mr. Han’s impeachment.

Mr. Choi, while acting president, filled two of the three vacancies on the court, but displeased the left by failing to put a ninth judge in the remaining seat.

This article has been updated from the bulldog edition.