The war of words spilled over into China’s state-controlled media, as the Global Times newspaper warned in an editorial that Chinese citizens would likely start campaigns to boycott American cars and other products.

Some American businesses in China also voiced concern. “Our members do not want to see a trade war,” said Kenneth Jarrett, the president of the American Chamber of Commerce in Shanghai. “The stakes are too high and there would be no winner,” Mr. Jarrett said.

But he added that American businesses in China wanted “fairer treatment and improved market access in China. The Chinese government has the ability to deliver against that reasonable expectation.”

In the United States, shares of large exporters, whose fortunes could be harmed by a trade war, were hit especially hard on Thursday and looked ready to bear the brunt of more selling on Friday. Shares of Boeing, one of the country’s largest exporters, and Caterpillar, which counts China as an important market, both fell by more than 5 percent.

Shares of large technology companies, which had already been reeling in anticipation of tougher government oversight, also took a hit. Facebook, which has been contending with a crisis over data privacy, slumped by more than 2 percent. Alphabet, Google’s parent company, dropped by more than 3.7 percent.

Amid the dip in stocks, money flowed to government bonds as investors sought safety, briefly driving yields on the benchmark 10-year Treasury note below 2.8 percent. Yields move in the opposite direction of bond prices. Commodities heavily geared toward global growth also fell. The price of West Texas intermediate crude oil, the American benchmark, slipped 1.2 percent. Copper, an important industrial metal, dropped 0.9 percent.

Thursday’s decline is the latest in a series of jolts to stock markets in the past two months.

After more than a year of calm, in which stock markets glided to one record high after another, a wave of volatility is suddenly cresting. There have been many causes of the turbulence this month and last. Investors initially were fearful that the economy was getting too strong, and that rising wages might cause inflation, which would push the Federal Reserve to hike interest rates faster than investors previously had expected. Those concerns have partly faded as recent economic data showed that inflation remained in check.