The protests over the death of George Floyd could lead to a new outbreak of coronavirus cases, experts warned.
Political leaders appealed for calm and had masks handed out to demonstrators, warning those without that they were putting themselves at risk.
Keisha Lance Bottoms, mayor of Atlanta, said: "If you were out protesting last night you probably need to go get a Covid test this week.
"There is still a pandemic in America that's killing black and brown people at higher numbers."
The protests came at a time when when many cities were beginning to relax stay-at-home orders.
Bradley Pollock, chairman of the Department of Public Health Sciences at the University of California, Davis, said: "Whether they're fired up or not that doesn't prevent them from getting the virus."
In Minnesota, where Mr Floyd died, the governor Tim Walz said too many protesters were not socially distancing or wearing masks.
"We are still in the middle of a pandemic and passed 1,000 deaths. We still have hospitals on the verge of being overrun with COVID-19," he said.
Barbara Ferrer, the Los Angeles County public health director, said: "Show respect for each other by putting on that face covering so that your respiratory droplets aren't unintentionally getting into somebody else's mouth, nose or eyes,"
Even for the many protesters who have been wearing masks, that does not guarantee protection from the virus, experts said.
The Centers for Disease Control and Prevention recommends cloth masks because they can make it more difficult for infected people to spread the virus, but they are not designed to protect the person wearing the mask from getting it.
Spence Ingram, 25, a protester wearing a mask in Minneapolis,, said: "It's not OK that in the middle of a pandemic we have to be out here risking our lives. But I have to protest for my life and fight for my life all the time."