"We have still not fully understood the degree of the economic impact," warned Massud Ghaussy, senior analyst at Nasdaq IR Intelligence in New York.
"Currently, from a policymaker's perspective, it's a relative balance between managing the spread of the virus and opening the economy."
After the market closed, President Donald Trump signed the stimulus package into law.
The bill, along with unprecedented policy easing by the Federal Reserve, helped the S&P 500 surge 10.2% for the week, its best week since 2009. But the U.S. stock market benchmark is still down about 25% from its February high.
In its strongest three-day performance since 1931, the Dow surged 21% in three straight days through Thursday, establishing it in a bull market, according to one widely used definition. Even after Friday's drop, the Dow ended 12.8% higher, its best week since 1938.
Many investors see a strong risk the market could fall deeply again as coronavirus infections increase and more people die, however.
"Next week will depend on what happens over the weekend," said Lindsey Bell, chief investment strategist at Ally Invest. "If there is a major acceleration over the weekend of coronavirus cases in New York and other states and the hospital system continues to get jammed up, then I think it will be a rough week for the market."
Macroeconomic indicators offered a glimpse of the economic devastation from the crisis as the lockdown of major cities upends the lives of millions of Americans.
U.S. consumer sentiment dropped to a near 3-1/2-year low in March, according to a survey released on Friday, a day after data showed a record 3 million surge in jobless claims last week.
The Dow Jones Industrial Average slumped 4.06% to end at 21,636.78 points, while the S&P 500 lost 3.37% to 2,541.47.
The Nasdaq Composite dropped 3.79% to 7,502.38.
Volume on U.S. exchanges was 13.4 billion shares, its lowest since March 5, according to Refinitiv data.
Delta Airlines, American Airlines and United Airlines fell between 6% and 11% as U.S. Treasury Secretary Steve Mnuchin said the help designated for airlines in the aid package was not a bailout and that taxpayers would need to be compensated.
Boeing Co slumped 10%, but was still up more than 70% for the week, after Mnuchin said the planemaker had no intention of using federal money.
The banking index fell 4.6%, tracking U.S. Treasury yields as investors sought safety in high-quality assets.
The energy index was the biggest percentage loser among the 11 major S&P sectors, sliding 6.9%, following a drop in oil prices.
Declining issues outnumbered advancing ones on the NYSE by a 3.17-to-1 ratio; on Nasdaq, a 2.98-to-1 ratio favored decliners.
The S&P 500 posted one new 52-week high and one new low; the Nasdaq Composite recorded nine new highs and 39 new lows.
(Reporting by Noel Randewich; Additional reporting by Uday Sampath and Medha Singh in Bengaluru; Editing by Daniel Wallis; Alistair Bell, Tom Brown and Richard Chang) ((email@example.com; (415) 677 2542; Reuters Messaging: Twitter: @randewich))