Americans ran to beaches and lakes on Memorial Day weekend, often ignoring restrictions imposed to combat the coronavirus outbreak.
In Florida, the state police dispersed an unauthorized meeting of hundreds in Daytona Beach on Saturday.
In Missouri, the bars on Lake Ozarks were full of revelers, who violated the rules of social detachment.
The head of the U.S. coronavirus task force, Dr. Deborah Birx, said she was "very concerned" after seeing these scenes.
"We really want to make it clear all the time that social distance is absolutely critical. And if you can't distance yourself socially and are outside, you should wear a mask," said Birx in ABC's This Week on Sunday .
Lyda Krewson, Mayor of St. Louis, Missouri, said: "It is irresponsible and dangerous to engage in high-risk behavior just to have fun over the long weekend.
"Now, these people will return home to S. Louis and Missouri and Midwest counties, raising concerns about the potential for more positive cases, hospitalizations and tragically deaths. Deeply disturbing."
In the Tampa area of Florida, the crowd was so large that the authorities closed the parking lots because they were full, reports the Associated Press.
In California, large crowds were seen enjoying beaches over the weekend. State officials said most people were covering their faces and keeping their distance on beaches and parks.
On Monday, US Vice President Mike Pence said that some people "would do well to be reminded of the importance of social detachment and the importance of maintaining regular hygiene".
However, he added, "We have every confidence that, as restrictions are loosened in the coming days, the American people will step forward and get this country back to work in a safe and responsible manner."
Memorial Day – an annual holiday held on the last Monday in May – honors all those who died serving in the U.S. military. It marks the unofficial start of summer.
The US has more cases of coronavirus than anywhere in the world. It has more than 1.6 million known infections and is reaching 100,000 deaths related to the virus.
All 50 US states have partially reopened after two months of stoppage. However, the remaining restrictions vary across the country.
There are also major discrepancies between states in terms of infection rates and the pace of their economic restart.
Some of the hardest hit areas, including the states of New York, New Jersey and Washington, are now experiencing the biggest declines, while most states have reached plateaus.