Worried officials are increasingly imposing curfews to try to quell the nightly unrest that has gripped American cities, a drastic measure that tacitly acknowledges how police departments have struggled with the size and intensity of protests over officer misconduct.

After several nights of violent clashes and fiery vandalism, officials in Washington, D.C., moved up their nightly curfew to 7 p.m., while New York City imposed its first nightly curfew since the protests began last week in response to the killing in police custody of George Floyd in Minneapolis. Dozens of U.S. cities have imposed curfews in recent days as they seek to contain the anger, violence and destruction spreading around the country.

“Curfews are the last resort, when police feel like they are out of options,” said Tamara Herold, a professor at the University of Nevada at Las Vegas who studies criminal justice and crowd management. “Curfews are a tool when things have become so chaotic that it’s no longer possible for police to protect our basic constitutional rights to free speech.”

The intensity of the unrest appears to have taken many police officials by surprise.

Unlike many other countries, Herold said, the United States does not have a national standard or model for dealing with mass protests that turn confrontational, and it seems as if many police departments were “caught off guard and are doing their best to restore order.”

A curfew telling citizens they cannot be on the street between certain hours — usually at night — is a blunt instrument to help police distinguish between peaceful protesters and those intent on harming officers or damaging property. The thinking behind imposing nighttime curfews is that they allow peaceful protesters to have their say and then leave. Those who don’t go home, the theory goes, probably intend to make trouble.

For that reason, curfews can be a signal to law enforcement to get more aggressive with whoever remains on the street, regardless of what those individuals are doing.

“So many cities are imposing curfews because so many cities are on the verge of rioting,” said Dennis Kenney, a former police officer who is now a criminal justice professor at John Jay College in New York. “We are one more video away from a lot of cities burning.”

The protests began last week after a video emerged of Floyd, a 46-year-old black man, pleading for air as a white Minneapolis police officer, Derek Chauvin, knelt on his neck for almost nine minutes. Floyd was taken to a hospital, where he was pronounced dead. Chauvin has been charged with third-degree murder and second-degree manslaughter, but protesters have demanded that charges also be filed against three other officers who were at the scene and did nothing to intervene as Floyd pleaded, “I can’t breathe.”

The expansion of curfews also comes as the federal government pushed city and state officials Monday to more aggressively pursue lawbreakers at such protests. Police in several cities already have ratcheted up their use of force — wielding batons, rubber bullets and pepper spray.

The FBI issued a statement Monday asking the public for any tips, videos or photos that might help them identify “violent instigators who are exploiting legitimate, peaceful protests and engaging in violations of federal law.”

In a conference call with governors, President Trump and Attorney General William P. Barr urged officials to move forcefully against the small number of criminals operating in the larger sea of law-abiding protesters, using military terminology to describe the situation.

“Law enforcement response is not going to work unless we dominate the streets,” said Barr, who has labeled the violence “domestic terrorism.” He added, “We have to control the crowds and not to react to what’s happening on the street, and that requires a strong presence.”

Defense Secretary Mark T. Esper told governors, “The sooner that you mass and dominate the battlespace, the quicker this dissipates.”