Markets dove Thursday amid fears President Trump’s new tariffs would start a global trade war, with the Dow Jones industrial average down more than 500 points at one point in midday trading and the Nasdaq Composite and the Standard & Poor’s 500 index down as well.

Markets have been shaky for several weeks since the president announced a 25 percent tariff on imported steel and a 10 percent tariff on imported aluminum. Trump on Thursday announced about $50 billion in annual tariffs on imports from China. The Chinese reportedly are preparing a tit-for-tat response by placing tariffs on U.S. agricultural products that have big markets in China, such as soybeans.

Investors are fearful the trade policies and their fallout could tank a robust global economy and hamper the nine-year bull market.

“Trade tariffs are starting to emerge as a bigger market head wind than originally thought,” said Ivan Feinseth, director of research at Tigress Financial Partners. “The strong U.S. and global economic and fiscal policy tail winds are starting to be overtaken by the proposed tariffs, the Fed’s softer-then-expected economic outlook for 2018 and the fallout from the Facebook issue.”

The Dow bounced back slightly shortly after it hit the 500 mark.

The Federal Reserve on Wednesday gave markets more bad news when it announced it would hike its interest rate, as the central bank continues to move away from the extraordinary efforts it has taken in the past decade to stimulate economic growth. Those moves have greatly boosted stocks since the 2008 financial crisis and ensuing Great Recession.

The Fed increased its projections for economic growth on Wednesday, bumping them up from previous estimates done before Republicans passed their tax bill. But the projections fall short of the sharp growth Republicans promised the tax cuts would create.

Tech stocks contributed to the decline as well.

Facebook, reeling from a data privacy scandal, has been a big drag on the tech-heavy Nasdaq during a hectic week for the sector. The F in the vaunted FAANG stocks — Facebook, Alphabet, Amazon.com, Netflix, Google — is down more than 2.5 percent in the last four trading days, and its losses are bleeding across the technology sector. Apple is down more than 1 percent Thursday, Amazon.com (whose chairman, Jeffrey P. Bezos, owns The Washington Post) was down 2 percent and Google-parent Alphabet dove more than 3 percent.

The embattled social media giant with more than 2 billion monthly active users has seen around 10 percent of its market capitalization, about $50 billion, vaporized this week on news that its vast trove of personal data had been misused by Cambridge Analytica, a data mining firm and political communications firm.

Facebook shares have been a key driver of the recent boom in technology, but it and other technology stocks have dropped on fears that regulators could impose rules that hinder their business models.

 The social media giant came under heavy fire from lawmakers in the United States and Britain over the weekend after news reports raised questions about whether it allowed third-party developers to access the data of users without their permission — a potential violation of its privacy agreement with the U.S. government.

Democratic Sens. Edward J. Markey and Amy Klobuchar said this week that they want Facebook co-founder and chairman Mark Zuckerberg to testify to Congress under oath about his company. Zuckerberg told CNN that he would be “happy” to address Congress.

“I’m really sorry that this happened,” he said during a CNN interview on Wednesday.