This protest at the Newseum was in response to President Trump’s comments after violence in Charlottesville. (Andre Chung/for The Washington Post)

In a city where protests are akin to a competitive — and sometimes combative — sport, Robin Bell has found a way to separate himself from the pack.

His light projections of pointed political jabs and commentary appear on the outside walls of buildings of power and influence, drawing attention from passersby and on social media. Then, with a flick of a switch, the words are gone — temporary graffiti that leaves its mark without trespass or stain.

Standing on public space, Bell and other activists have projected on the FBI headquarters, D.C. Superior Court and the Trump International Hotel, where they displayed “Pay Trump bribes here” over an entrance.

For the first time in years of such activism, one of the projectionists, Robby Diesu, was arrested Wednesday night, as he displayed the words “discrimination is wrong” onto the side of the Rayburn House Office Building, a seemingly gentle reminder to lawmakers debating the Equality Act, which would ban discrimination against the ­LGTBQ community.

Bell said people involved in the efforts have interacted with D.C. police and other agencies at previous displays and have never been arrested or asked to stop.

But the Capitol Grounds, protected by U.S. Capitol Police, falls under its own set of regulations which map out space around buildings where protests are — and aren’t — allowed. Police say Diesu was in an area where demonstrations are not allowed, and that projecting onto buildings is barred generally.

Diesu’s arrest on a misdemeanor charge could force debate over high-tech ways of protesting and laws governing free speech and trespass.

“If the reason for the arrest is that the government didn’t like the message, then that would raise major First Amendment, free speech concerns,” said Clay Calvert, a professor and director of the Marion B. Brechner First Amendment Project at the University of Florida. “If the government has a content neutral reason for preventing images then that is more likely to be upheld. I think this is probably their message, that a government building is not a billboard for people’s speech.”

Diesu, 30, was detained for about two hours as police confiscated his expensive projection equipment and stowed it on the back of a white pickup truck. He was given a citation for allegedly violating a regulation that restricts demonstrations on the steps of “any building on Capitol Grounds or in any area otherwise closed or restricted for official use.”

The arresting officer wrote Diesu had violated a regulation “prohibiting the projection of images.”

Capitol Police say the prohibition against projecting images on the sides of congressional buildings was always implicit in the regulation. But in February, officials added new language that explicitly forbids the “projecting of images.”

Diesu said he believes he was in a restricted protest area but that he has previously protested at the doors of the Capitol building without incident. “It seems the arrests are arbitrary,” he said. “That’s problematic.”

Experts in criminal law say one issue for the courts could be whether the prohibitions go too far, infringing on people’s First Amendment right to protest.

“We have a long history of protests on the Capitol grounds, and the court should consider this when deciding whether restricting Mr. Diesu’s rights in this way truly served a significant governmental interest,” said Steven J. McCool, a former prosecutor and now defense lawyer in Washington who successfully represented one of the demonstrators charged with rioting at President Trump’s inauguration.

“If Mr. Diesu projected his message from a public street onto a private building, it would be interesting to see if he could be prosecuted for trespassing,” McCool said. “Usually, a trespass occurs where there is a physical intrusion on private property. . . . Courts are only recently facing the issue of whether projected images and messages can amount to a trespass.”

McCool said it’s easy to be sympathetic to Diesu “because we all should agree that discrimination is wrong,” but people should consider that the method could also be used to project “a message of hate.”

A state appeals court in Nevada confronted the issue after members of a union projected messages on a restaurant during a labor dispute. The restaurant owner argued the union had trespassed and altered the appearance of the building.

The court rejected the argument, but one judge, in a concurring opinion, said the building owner’s might have had a better argument that the projected images presented a nuisance and interfered with their property rights.

“Fundamentally, the problem here is that we are confronted with a clash between very old law and evolving new technology,” Nevada Court of Appeals Judge Jerome Tao wrote, referring to the projections as “light invasions.”

But while the Nevada case involved projecting an image onto a private building, Diesu’s message appeared on a public building that embodies the very essence of debate.

R. George Wright, a professor in the Robert H. McKinney School of Law at Indiana University, said the courts may consider whether the protest interferes with government or someone else’s rights, or if it poses a security risk. Unlike some demonstrations that may be noisy, block entrances or cause disruption, the projections “aren’t doing any physical damage or permanently affecting the people. It’s just photons that bounce off. If I switch off the machine, it’s as if nothing happened.”

Bell said he began his light-based protest movement many years ago, but he ramped it up under the Trump administration. “I feel we are challenging what the First Amendment is and creating conversations,” he said.

This story has been updated.