NEW YORK — Outside Brooklyn's Barclays Center, thousands of protesters churned this weekend in tightly packed crowds, casting aside social distancing to express their rage and grief.

In Minneapolis, ungloved demonstrators held hands as they marched.

In Las Vegas, demonstrators roared their anger into the faces of police lined up just a few feet away.

And in nearly two dozen U.S. cities, police grappled physically with more than 2,500 people arrested during often-violent protests over the death of a black man, George Floyd, in the custody of Minneapolis police on Memorial Day.

The rules of the covid-19 pandemic, so recently learned at considerable inconvenience, have been discarded on the streets in recent days. Protesters frequently find it impossible to stay six feet apart, to avoid hand-to-hand contact or to dodge the respiratory droplets of their shouting, chanting comrades amid the swirling chaos. And because the virus can be spread by people with no symptoms, it can be impossible to figure out whom to avoid.

Officials are clearly worried about the possible impact of the protests on the health crisis. As of Sunday, the United States had recorded 1.7 million coronavirus infections and 103,000 covid-19 deaths — a disproportionate number of them black and brown people.

D.C. Mayor Muriel E. Bowser (D) said she is concerned about renewed outbreaks caused by large demonstrations in the nation’s capital. And Atlanta Mayor Keisha Lance Bottoms (D) urged her city’s demonstrators to seek tests for the virus as soon as possible.

“If you were out protesting last night, you probably need to go get a COVID test this week,” Bottoms said at a news conference Saturday. “There is still a pandemic in America that’s killing black and brown people at higher numbers.”

Experts said it remains to be seen whether the protests will produce a surge in infections. Given the behavior on the street, they said, there is cause for concern.

“Crowded protests, like any large gathering of people in a close space, can help facilitate the spread of covid-19, which is why it’s so important participants wear masks, eye protection and bring hand-gel,” Saskia Popescu, an infectious-disease epidemiologist at George Mason University’s Schar School of Policy and Government, wrote in an email.

“Shouting and screaming, as some studies have shown with singing, can project droplets farther, which makes the use of masks . . . and eye protection . . . that much more important.”

On the other hand, several circumstances play to the protesters’ benefit, experts said. The most critical is being outside; open space and breezes dilute and disperse the virus. In one study — which has yet to be peer reviewed — of 7,324 infected people in China, only two contracted it outdoors.

Researchers in Japan who looked at a small number of cases concluded that the chances of transmitting the virus are nearly 19 times greater indoors than out. That study also is awaiting review by other scientists.

“There are so many variables at play here: Extent of social distancing, ambient environmental conditions, number of people, extent of mask use, the effect of things like tear gas [and] pepper spray on susceptibility via different transmission routes,” Angela Rasmussen, a virologist at the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health, wrote in an email.

“I don’t think there’s any way to know how bad it will be,” she added, “but there is likely to be increased cases in cities with large protests.”

Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, who specializes in airborne transmission of viruses, said that even when people are crowded against each other, it takes time to transmit the virus in significant amounts — especially outdoors. She said she would worry if the density of the crowd approximated the conditions of packed seats in a basketball arena, and if people did not move much for at least a half-hour.

Mask-wearing by infected people would cut down on the spread of respiratory droplets, offering some protection to people nearby. Unless they are rated N95 or better, however, masks offer only limited protection against the microscopic virus for the people wearing them.

On the streets in recent days, many protesters, police and reporters appeared to be wearing masks, though some did not. Some police officers also wore plastic face shields.

“Outdoor contact is far, far less risky than indoor contact,” said Tom Frieden, former head of the Centers for Disease Control and Prevention. “When outdoors within 6 feet, a mask will further reduce risk.”

Shouting projects droplets farther than speaking, however. The best-known incident of this means of infection came in March, when 53 members of a Washington state choir were infected during a single rehearsal by droplets expelled while they sang. Two of them died.

Even ordinary speech can send out droplets that carry virus. One research group found these can linger for eight minutes — and possibly much longer — in stagnant air under laboratory conditions. The study could help explain why infections so often occur in houses, nursing homes, conferences, cruise ships and other confined spaces with limited air circulation.

The report, from researchers at the National Institute of Diabetes and Digestive and Kidney Diseases and the University of Pennsylvania, was published in May in the Proceedings of the National Academy of Sciences, a peer-reviewed journal.

Frieden raised one other aspect of the protests that may contribute to the spread of the virus: the breakdown of trust in government. “Successful public health requires engagement and trust of the community,” he said.

Some demonstrators said they weighed the risk of covid-19 when deciding whether to attend a protest. Columbia University student Juliet Shatkin, 26, said her friends were “nervous about coronavirus,” so they did not join her during a protest Saturday on Manhattan’s Upper East Side.

“It’s scary, but I don’t know,” Shatkin said. “People are mad, and everyone I’ve seen is wearing a mask.”

Elise Barr, a teacher at a child-care center, wore a mask to attend a protest Sunday in Kansas City, Mo., but she said she was not worried about catching the virus.

“Coronavirus is going to have to take a back seat. This movement is about more than that,” she said. “Black people are being murdered.”

Shayna Jacobs, Annie Gowen, Joel Achenbach and Meryl Kornfield contributed to this report.