In a series of angry tweets earlier in the day, Mr. Trump called for American companies to cut ties with Beijing and said the United States would be economically stronger without China. Those comments sent stocks plunging, helping push the market to its fourth straight weekly loss. The president also called the Fed chair, Jerome H. Powell, an “enemy” of the United States and compared him to President Xi Jinping of China, his trade nemesis, after Mr. Powell declined to signal an imminent cut in interest rates.

Mr. Trump has been counting on Mr. Powell to help blunt the effect of his trade war by cutting interest rates to keep the economy humming. While Mr. Powell said on Friday that the Fed could push through another cut if the economy weakened further, he suggested that the central bank’s ability to limit economic damage from the president’s trade war was constrained.

“My only question is, who is our bigger enemy, Jay Powell or Chairman Xi?” the president tweeted.

Mr. Trump’s tariff announcement came after financial markets had closed for the day. But his earlier response to China had already unnerved investors, who worry that the trade war between the world’s two largest economies will further drag on global growth. Stocks fell sharply on Friday, with the S&P 500 closing down 2.6 percent. The Dow Jones industrial average was down slightly more than 2 percent and the technology-heavy Nasdaq index fell 3 percent.

Behind the tirade was the growing reality that the type of trade war Mr. Trump once called “easy to win” is proving to be more difficult and economically damaging than the president envisioned. Mr. Trump’s stiff tariffs on Chinese goods have been met with reciprocal levies, hurting American farmers and companies and contributing to a global slowdown.

On Friday, China said it would increase tariffs on $75 billion worth of American goods, including crude oil, automobiles and farm products like soybeans, pork and corn in response to Mr. Trump’s plan to tax an additional $300 billion worth of Chinese goods in September and December.