The rout in stock markets continued on Friday as worries deepened about a global trade war, after China retaliated against President Trump’s sweeping tariffs with steep levies of its own on U.S. goods.

The S&P 500 was set to open more than 3 percent lower on Friday, according to futures trading. On Thursday, the index posted its worst daily loss since 2020, plunging 4.8 percent.

European and Asian indexes also fell sharply for a second day as investors weighed the economic effects of Mr. Trump’s tariffs. The Stoxx Europe 600 dropped more than 4 percent, erasing its gains for the year. In Japan, the Nikkei 225 fell 2.8 percent, matching a drop the day before.

The Chinese government said on Friday that it would match Mr. Trump’s plan for 34 percent tariffs on goods from China with its own 34 percent tariff on imports from the United States. It also added 11 American companies to its list of “unreliable entities,” essentially barring them from doing business in China or with Chinese companies.

The moves showed that Beijing has no intention of backing down in the trade war with Mr. Trump. Investors were already on edge because of the potential disruption that Mr. Trump’s policies, including widespread deportations and layoffs of federal workers, could have on the economy.

His announcement on Wednesday, which he called “Liberation Day,” included a minimum tariff of 10 percent on nearly all imports, causing markets to convulse as investors dumped stocks and sought safety in government bonds.

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