SEOUL — Americans, watch out: You’re in far greater danger this week if you’re in central Seoul than in the demilitarized zone facing North Korean soldiers across the line or at military bases that the North Korean leader, Kim Jong-un, threatens to wipe out with his nukes and missiles.

That’s the implication of a message posted on X by the American embassy at Seoul telling American citizens to “avoid areas where demonstrations are taking place” and “exercise caution” anywhere near “large crowds, gatherings, protests or rallies.” The reason for the extraordinary warning is the mounting tension over the decision coming down from South Korea’s constitutional court on Friday on whether to approve the impeachment of the country’s conservative president, Yun Suk-yeol.

China and Russia, at odds with America on issues ranging from Ukraine to Taiwan, agree: They’ve put out similar warnings, with the Chinese telling its citizens not to “stay near, participate in or watch” protests, according to Seoul’s Yonhap News.

Although protests by hundreds of thousands of Koreans have been largely nonviolent, both right and left are threatening rebellion if the court decision does not go their way. Hotheads on the far left blame the historic Korean-American alliance for impeding reconciliation with North Korea, while rightists demonstrators, waving American and Korean flags, accuse the leftists of being “pro North” and under the influence of Communist China, reviled as influencing and infiltrating South Korean society, its political system, and government.

The right-left standoff has been mounting, with emotions scarcely pent-up ever since Korea’s national assembly, dominated by the opposition Democratic, or Minju, party, quickly rejected a martial law decree that Mr. Yoon sought to impose on December 3 and then voted to impeach him 11 days later. The vote stripped him of his power but not the title of president. If the constitutional court approves the impeachment decree, he’s ousted as president; if the decree is rejected, he assumes full authority as before.

“U.S citizens should anticipate large-scale demonstrations and an increased police presence,” the embassy warned in an understatement of the precautions announced by Seoul’s acting police chief, Lee Young-ho, filling in for the police commissioner and police chief. Those two are in jail along with a few dozen others, including a former defense minister and a raft of generals charged with “insurrection” for having supported the martial law decree. Mr. Yoon, freed from jail,  faces the same charge regardless of his status as president.

Yonhap reports that Mr. Lee has “mobilized 100 percent of the police force, about 14,000 officers from 14 units,” to “prepare for unexpected situations.” Police buses are blocking off traffic on streets around the court, and nearby subway stations are closed to stop mobs of demonstrators from both sides from getting anywhere near the building. Thousands more will be on high alert in other major cities while the entire country awaits the news from the court, to be broadcast live on all Korean TV and radio networks.

It’s just at that revelatory moment when tensions are most likely to burst as demonstrators, massed on avenues beyond the police buses, stand transfixed before enormous screens set up on trucks from which they’ve been hearing speeches and patriotic music at regular demonstrations for months.

“It is feared that after the ruling some in the crowd may stage extreme and violent protests leading to accidents,” Yonhap quoted Mr. Lee as saying. “Full police powers will be mobilized to prevent serious social conflict.”

Those remarks easily eclipsed what many people see as fairly routine statements by senior military officers of the danger posed by North Korea — or, for that matter, threats by Mr. Kim to dip into his growing inventory of nukes and missiles to attack the South.

A warning of the “immediate security challenge” posed by North Korea’s “long-range missile and nuclear programs” received little attention here even though the source was Mr. Trump’s choice for Armed Forces chief of staff, John Caine, a retired Air Force lieutenant general. Nor did a North Korean warning about deploying its “powerful deterrence” against American and Japanese cooperation on military exercises and weapons development generate much interest beyond secondary items in the press.

The main consuming topic was that of the outcome of Korea’s ongoing political crisis. People with whom I spoke seemed to have definite opinions as to the result. Those who oppose Mr. Yoon were sure he would soon be ousted — and then face trial for insurrection. Mr. Yoon’s diehard advocates had no doubt he would be back at his desk as president, and then live up to his promises to revive the constitution so his foes could not undermine the powers of the president so easily.

As a favorable sign, conservatives cited the constitutional court’s rejection last month of an impeachment motion passed by the assembly against the acting president, Han Duck-soo, who was prime minister when Mr. Yoon was impeached and then filled in for him. Mr. Han, having been exonerated by the assembly, is now back as acting president. 

A career bureaucrat and former ambassador to Washington, Mr. Han is trying to tamp down emotions. “We must accept the result in a calm and level-headed manner,” he pleaded. 

If Mr. Yoon is ousted, Mr. Han will remain as acting president through a “snap election” to be held within 60 days and then for another month or two before the new president is inaugurated. That is, unless Mr. Han ‘s foes in the Minju manage to impeach him again, as they have about 30 others in Mr. Yoon’s government. Whatever happens Friday, the struggle for leadership is sure to go on — with unpredictable results.