By Gunjan Banerji and Caitlin Ostroff
Declines in shares of technology companies pushed the S&P 500 toward a third consecutive week of declines, capping another tumultuous stretch for the stock market.
The S&P 500 slipped about 1.5%, with losses accelerating midday. The tech-heavy Nasdaq Composite lost 1.8%. The Dow Jones Industrial Average shed about 326 points, or 1.2%.
The S&P 500 and Nasdaq are on track for weekly declines, while the Dow is headed for a slim gain.
Friday's moves continue a spell of volatility that has dominated recently, mainly driven by the tech heavyweights that powered the market higher this year. These companies continued to falter Friday, dragging major indexes lower.
The big tech companies have outsize influence on the S&P 500 given their towering market valuations.
In one sign of the reversal at play in recent weeks, a version of the S&P 500 that gives every company an equal weighting has barely budged this month. Meanwhile, the standard index has slipped about 5.1%.
Stocks have also swung this week as the Federal Reserve said the U.S. economic outlook remains highly uncertain. Some investors were disappointed that the central bank didn't offer more guidance around additional stimulus measures or specifics about its inflation targets.
Speculation that the Fed would keep interest rates near zero for some years, resulting in low bond yields, has been encouraging many people to move funds into riskier assets like stocks.
The occurrence of so-called quadruple witching -- when both futures and options linked to individual stocks and stock indexes expire on the same day -- means investors are braced for another choppy session Friday. A surge in options trading targeted at giant technology stocks by both small and large investors has also been magnifying the market's ups and downs in recent days.
"It is normally a day where you're glued to your screen and you're watching for volatility," said Altaf Kassam, head of investment strategy for State Street Global Advisors in Europe. "We've had an abnormal amount of options on single stocks, so you might see some movement in single stocks."
In corporate news, shares of Oracle lost 0.6% after the Trump administration said it would block downloads of TikTok and the use of Chinese messaging and payment app WeChat. Under the latest plan for a majority U.S. ownership of Chinese-owned video-sharing app TikTok, Oracle and Walmart could together own a significant stake.
Shares of Nvidia lost 2.7%, while Amazon.com stock fell about 2.5%.
Investors are also watching for progress in Covid-19 vaccine trials and monitoring economic data to assess how much stocks can climb, following a rebound from March lows.
"If you think about it, we've had all the easy gains," said Mr. Kassam. "It's going to get incrementally harder for us to keep pushing up that hill."
Fresh data released Friday showed that consumer sentiment in the U.S. increased more than expected in early September, though confidence among Americans remained at depressed levels.
Also on Friday, lawmakers are aiming to unveil a bipartisan spending bill averting a government shutdown next month. Democrats and Republicans remain at an impasse over another round of coronavirus relief despite President Trump's renewed interest in a deal.
The breakdown in negotiations has disappointed some investors who expected another relief package would lend more spending power to Americans and boost economic recovery. Additional support could bolster consumer spending, which powers two-thirds of the U.S. economy.
Overseas, the pan-continental Stoxx Europe 600 fell 0.7%.
In Asia, China's Shanghai Composite Index led regional gains, with a more than 2% rise by the close of trading, helped by rallies in financial stocks. State-owned China Life Insurance and New China Life Insurance both surged by 10%, the maximum daily gain allowed, as did broker Zheshang Securities.
Foreign investors have added to their holdings of Chinese stocks, helping bolster the yuan, which in recent days has hit its strongest levels against the dollar since May 2019.
Elsewhere, major indexes in Hong Kong, Japan and South Korea advanced less than 0.5%, while the Australian benchmark fell slightly.
--Joanne Chiu in Hong Kong contributed to this article.
Write to Gunjan Banerji at Gunjan.Banerji@wsj.com and Caitlin Ostroff at firstname.lastname@example.org