President Trump’s decision to impose sweeping tariffs on some of America’s largest trading partners sent shock waves through markets across the globe on Monday.

The dollar strengthened, oil prices rose and major stock indexes in the United States fell at the start of trading on Monday, with the S&P 500 down 1.6 percent, and the technology-heavy Nasdaq down 2 percent. Markets in Asia and Europe also tumbled.

When Mr. Trump was elected, many analysts and investors had brushed off his more aggressive tariff talk as bluster intended to prompt negotiation from his global counterparts. But over the weekend, the new administration followed through on the president’s promise to impose tariffs of 25 percent on imports from Canada and Mexico, the United States’ closest trading partners. Canadian energy products and goods from China will be levied at 10 percent. The tariffs are set to go into effect on Tuesday.

The drop in the S&P 500 “seems broadly justified as tariff fears rise,” said Yung-Yu Ma, the chief investment strategist for BMO Wealth Management. “The uncertainty at this stage is tremendous — not only of how these eventual negotiations will play out, but worries about how this is only the tip of the iceberg and more tariffs are on the horizon.”

Shortly after Mr. Trump’s announcement this weekend, leaders in Canada and Mexico said they would respond by placing retaliatory tariffs on U.S. goods. The peso and Canadian dollar both declined as the U.S. dollar strengthened.

Asian and European stock markets also slid on Monday. Japan’s Nikkei 225 index and South Korea’s Kospi each fell more than 2.5 percent. Markets in mainland China were closed on Monday for the Lunar New Year holiday. The Euro Stoxx 50, made up of Europe’s largest companies, dropped 2 percent.