U.S. stocks closed sharply lower Friday, with the Dow Jones Industrial Average tumbling as much as 357 points in afternoon trading Friday, amid jitters over Greece’s debt crisis and concerns over regulations in China. Investors also weighed the strengthening dollar's drag on corporate profits during earnings season.

All sectors were negative Friday, with financials, energy and technology among the biggest laggards.

The Dow (INDEXDJX:.DJI) plunged 279.47 points, or 1.54 percent, to close at 17,826.30. The Standard & Poor's 500 (INDEXNASDAQ:.IXIC) dropped 23.81 points, or 1.13 percent, to end at 2,081.18. The Nasdaq composite (INDEXSP:.INX) lost 75.98 points, or 1.52 percent, to finish at 4,931.81. 

For the week, the Dow dropped 231 points, or 1.28 percent, and the S&P 500 lost 20.88 points, or 0.99 percent. The Nasdaq lost 64.17 points, or 1.28 percent.

First-quarter earnings season has been forecast to be weak, as S&P 500 profits are expected to decline 1.5 percent from a year ago, their worst showing since 2009, according to research firm Estimize. Investors are becoming more concerned about corporate earnings as many companies report revenue shortfalls, including a trio of mixed reports on Friday from General Electric Company, Honeywell International Inc. and American Express Company.

"I have been amazed at the market’s ability to ignore all of the warning signs that have been flashing over the last few weeks to keep it marching higher," said Keith Bliss, senior vice president and director of sales and marketing at Cuttone & Co. Inc., citing a weak forecast for earnings season, tensions in Greece and an economic slowdown in China. 

The S&P 500 financial sector lost 1.3 percent Friday, with American Express Company (NYSE:AXP) dropping more than 4 percent after the world's largest credit card issuer posted quarterly revenue of $7.95 billion, below forecasts, due to a strong dollar. The S&P 500 energy sector lost nearly 1 percent Friday, with London-based oil and gas company Noble Corp. plc (NYSE:NE) and deepwater contractor Diamond Offshore Drilling Inc. (NYSE:DO) among the biggest decliners, dropping 3.1 percent and 2.6 percent, respectively.

Meanwhile, the S&P 500 information technology sector dropped 1.4 percent, with Salesforce.com Inc. (NYSE:CRM) among the largest decliners, falling more than 3 percent. Internet giant Yahoo! Inc. (NASDAQ:YHOO) and e-commerce company eBay Inc. (NASDAQ:EBAY) fell 2.9 percent and 2.1 percent, respectively.

The European Commission announced Friday that Athens must give finance ministers a concrete set of reforms by May 11, as Greece is on the verge of default.

Meanwhile, China’s regulators tightened rules on margin lending Friday, cracking down on over-the-counter margin trading and regulations that allow fund managers to lend shares for short-selling. China, the world’s second-largest economy, announced Wednesday its grew at its slowest pace in six years last quarter, signaling more monetary stimulus could be expected soon. Gross domestic product grew an annual 7 percent in the first quarter, slowing from 7.3 percent in the fourth quarter of 2014, the National Bureau of Statistics said Wednesday.

Ahead on the earnings calendar, investors are awaiting quarterly results next week from notable companies, including global financial services company Morgan Stanley, Internet giant Google Inc., social networking company Facebook Inc. and software company Microsoft Corp.