After months of rallying and periods of relative calm, stocks tumbled on Friday as fresh economic data reflected unexpected signs of weakness in the labor market and President Trump announced steep new tariffs against some of America’s largest trading partners.

The S&P 500 ended the day down 1.6 percent, capping one of the index’s worst weeks since Mr. Trump wrought chaos across the global trading system when he unveiled his first round of steep tariffs in April. The benchmark fell 2.4 percent for the week.

On Friday, investors parsed through the president’s latest tariff plans and how they might further drive up costs for companies and consumers. But it was a report from the Labor Department that caused the most alarm.

U.S. employers added 73,000 jobs in July, fewer than the roughly 100,000 that economists had expected, and the unemployment rate rose slightly. The report also revised down the data on hiring from May and June by a combined 258,000 jobs, suggesting the labor market was under greater strain than initially believed.

The weaker-than-expected hiring data, particularly the large downward revisions for May and June, raised concerns about the strength of the economy under Mr. Trump and created new uncertainty about the timing of the Federal Reserve’s next interest rate cut.

With the stock market swooning and his critics raising new questions about the efficacy of his economic policies, Mr. Trump took the extraordinary step of publicly calling into the question the veracity of the hiring data. In a social media post on Friday afternoon, he blamed, without evidence, a Biden administration appointee in the Bureau of Labor Statistics for producing faulty numbers.