Many Americans visited beaches for the Memorial Day weekend amid concerns from officials that the crowds could result in a spike of coronavirus cases.
In Florida, state police dispersed an unauthorised gathering of hundreds of people in Daytona Beach on Saturday.
In Missouri, bars at the Lake of the Ozarks were packed with revellers, who violated social-distancing rules.
US coronavirus task force chief Dr. Deborah Birx said she was “very concerned” after seeing such scenes.
“We really want to be clear all the time that social distancing is absolutely critical. And if you can’t social distance and you’re outside, you must wear a mask,” Dr Birx said on ABC’s This Week on Sunday.
Lyda Krewson, the mayor of St Louis, Missouri, said: “It’s irresponsible and dangerous to engage in such high risk behaviour just to have some fun over the extended holiday weekend.
“Now, these folks will be going home to S. Louis and counties across Missouri and the Midwest, raising concerns about the potential of more positive cases, hospitalizations, and tragically, deaths. Deeply disturbing.”
In California, big 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 about the importance of social distancing and the importance of continuing regular hygiene”.
However, he added: “We have every confidence that as restrictions are loosened in the days ahead, the American people are going to step forward and put this country back to work in a safe and responsible way.”
Memorial Day – an annual holiday held on the last Monday of May – honours all those who have died serving in the US military. It marks the unofficial start of summer.