Stocks were routed again Friday on Wall Street — a brutal finish to the worst week in the market in almost four years.

Investors were worried about signs of a slowdown in the Chinese economy that could hammer companies and countries around the world. The stock of Apple, which depends heavily on demand from China, fell more than 6 percent.

The Dow Jones industrial average plunged more than 500 points, the first time since August 2011 that it has taken such a steep fall in a single day.

It also reached a 10 percent decline from its all-time high — the definition of a market correction. That has not happened in four years.

Just after the market close, the Dow was down 528 points at 16,462. The average has lost more than 1,100 points in August alone.

The Standard & Poor’s 500 index, a broader gauge of the stock market, was down more than 3 percent and closed below 2,000 for the first time since early this year.

“Right now there is a feeling of fear in the marketplace, and all news is interpreted negatively and it’s interpreted indiscriminately,” Tom Digenan, the head of U.S. equities at UBS Global Asset Management, told CNBC.

The damage wasn’t limited to Apple. Energy stocks and technology stocks were hammered across the board. Utility stocks, seen as a relatively stable bet in a rough market, fared the best, but even they were down 1 percent.

There was some good news in the sell-off: The price of crude oil plunged below $40 a barrel for the first time since the financial crisis in 2009. Gas has already fallen to a nationwide average of $2.63 a gallon — 80 cents lower than a year ago.

Stocks began falling last week when China announced that it was devaluing its currency, the yuan. Investors took it as a signal that the slowdown in the Chinese economy, the second-largest in the world, is worse than they had thought.

They got more evidence on Friday: A reading of business activity in China, known as the Caixin purchasing index, fell to 47.1. Anything below 50 suggests business activity is shrinking.

“China has been on a mission to keep up the illusion of a gradual slowdown, but dealers aren’t buying it anymore,” David Madden, a market analyst at the financial company IG, told The Associated Press.

The punishment for investors was global. Earlier in the day, the Shanghai stock market fell 4.3 percent, and the Nikkei index in Japan tumbled 3 percent to its lowest level in six weeks. European stocks fell more than 3 percent.

Robert Shiller, a Nobel Prize-winning economist who has warned for months that the market was overheated, told CNBC that “even bigger and bigger moves” could be in the offing.

He said he was confident that the Chinese economy would grow and said that the American housing market “has been going up at a good clip.” But he warned that the market sell-off could spiral.

“When people who don’t normally pay attention to the market are brought in,” he told CNBC, “it can feed on itself like an epidemic.”