People packed bars and restaurants in Missouri and beaches in Georgia and Florida, while other places remained closed or followed social distancing rules
All 50 states have partially reopened with varying degrees of restrictions.
Dr Deborah Birx, the US coronavirus taskforce chief, urged people to continue to wear masks if they couldn't appropriately social distance.
She added that she was "concerned" by the crowded scenes.
Big crowds turned out for the Memorial Day weekend in the US amid warnings from authorities about people disregarding the coronavirus social distancing rules and risking a resurgence of Covid-19 as the country approaches 100,000 deaths from the illness.
The US is on track to mark the grim milestone in the next few days, while Europe has seen over 169,000 dead, according to a tally by Johns Hopkins University that almost certainly understates the toll. Worldwide, more than 5.4 million people have been infected and nearly 345,000 have died.
Dr Deborah Birx, coordinator of the White House coronavirus task force, said she was “very concerned” about scenes of people crowding together over the weekend.
“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,” Birx said on ABC’s This Week.
But across the US there was a patchy picture as many places remained closed or abided by strict social distancing rules.
Memorial Day in New York City is being marked with car convoys and small ceremonies instead of parades, as those looking to honor fallen military members have to contend with coronavirus restrictions.
“It’s something we’re upset about, but we understand,” said Raymond Aalbue, chairman of the United Military Veterans of Kings County, which usually puts on a parade in Brooklyn.
There’s “no reason to put anybody in harm’s way”, he said. Still, “it’s really cutting quick to the heart of all the veterans.”
His organization will have a convoy of 10 cars leaving from the Bay Ridge spot where the parade usually starts and then ending up at the Brooklyn veterans affairs hospital, where participants will line up next to their cars for a salute and a wreath-laying ceremony.
In Missouri, people packed bars and restaurants at the Lake of the Ozarks, a vacation spot popular with Chicagoans.
Missouri’s health director, Randall Williams, issued a warning on Monday after photos and video showed weekend revelers partying close together. One video on social media showed a crammed pool at Lake of the Ozarks, with people lounging and playing without masks. Williams said in a statement that such behavior could have “long-lasting and tragic” results.
On Georgia’s Tybee Island, the beach was filled with families, but at a nearby grocery store, staff members handed customers gloves and a number to keep track of how many people were inside.
In California, beaches and parks were open for swimming, running and other activities.
At New York’s Orchard Beach in the Bronx, kids played with toys, and people sat in folding chairs. Some wore winter coats on a cool and breezy day, and many wore masks and sat apart from others.
“Good to be outside. Fresh air. Just good to enjoy the outdoors,” said Danovan Clacken, whose face was covered.
The issue of wearing masks in public and staying several feet apart has become fraught politically, with some Americans arguing that such rules violate their rights.
Republican Governor Mike DeWine of Ohio, who has been targeted by such demonstrations, insisted the precautions should not be a partisan issue.
“This is not about whether you are liberal or conservative, left or right, Republican or Democrat,” he said on NBC’s Meet the Press.
The Trump administration said Sunday that it would ban foreign nationals who have been in Brazil 14 days or less before planning to enter the United States. The ban does not apply to US citizens or legal permanent residents or some of their relatives. Brazil is second only to the US in reported coronavirus cases.