By Caitlin Ostroff and Xie Yu

U.S. stocks tumbled Thursday as heightened fears of a second wave of coronavirus infections sent investors out of risky assets.

The Dow Jones Industrial Average fell 875 points, or 3.2%, shortly after the opening bell. The S&P 500 lost 2.5% and the Nasdaq Composite declined 2.1%.

U.S. coronavirus cases have topped 2 million, with the death toll climbing past 111,000. Data from Johns Hopkins University showed an uptick in cases in Florida, Texas and California, raising alarms that a second wave of infections could lead to restrictions being applied again on social and business activity.

"If we see those numbers coming back in the biggest economy in the world, then that's going to lead to a massive correction," said Altaf Kassam, head of investment strategy for State Street Global Advisors in Europe.

The cautious tone the Federal Reserve struck Wednesday has also been weighing on investors' minds, Mr. Kassam said. The Dow declined 1% Wednesday after Fed Chairman Jerome Powell said the coronavirus pandemic could inflict long-run damage on the economy by changing consumer and business behavior, and the labor market may not recover for "some years."

"Because Powell was incredibly dovish, it's finally settled that we're in this for the long haul. We're not going to recover quickly and there will be pain ahead," he said.

While investors don't expect broad-based lockdowns to be reinstated, regional infection increases in the U.S. could prompt local measures to curtail the spread, which will weigh on economic recovery, said Hani Redha, a multiasset portfolio manager at PineBridge Investments.

"There are some areas that are looking concerning, and that is one of the things that is going to get the market's attention," he said.

Equities retreated in international markets too, with the pan-continental Stoxx Europe 600 dropping 3%.

Yields on European government bonds fell as eurozone finance ministers met to discuss a proposed rescue package to fund the region's recovery. Investors' expectations for the size and scope of the aid package have dimmed as some EU members have pushed back against an ambitious plan recently floated by the European Commission.

Markets aren't pricing in the likely disagreement that the proposal will face from some member nations, said James Athey, senior investment manager at Aberdeen Standard Investments.

Gold, which is also widely perceived as a haven asset during times of market turbulence, climbed 1.5%.

In the Asia-Pacific region, Hong Kong's Hang Seng Index fell 2.3% by the close of trading. The benchmark in Japan dropped by 2.8%, and the Australian gauge fell 3.1%.

Akane Otani contributed to this article

Write to Caitlin Ostroff at caitlin.ostroff@wsj.com and Xie Yu at Yu.Xie@wsj.com