For most of his 40-year career, Carlos Moreno, a scientist and business professor in Paris, worked in relative peace.

Many cities around the world embraced a concept he started to develop in 2010. Called the 15-minute city, the idea is that everyday destinations such as schools, stores and offices should be only a short walk or bike ride away from home. A group of nearly 100 mayors worldwide embraced it as a way to help recover from the pandemic.

The conspiracy theorists came late, but suddenly.

In recent weeks, a deluge of rumors and distortions have taken aim at Mr. Moreno’s proposal. Driven in part by climate change deniers and backers of the QAnon conspiracy theory, false claims have circulated online, at protests and even in government hearings that 15-minute cities were a precursor to “climate change lockdowns” — urban “prison camps” in which residents’ movements would be surveilled and heavily restricted.

Many attacked Mr. Moreno, 63, directly. The professor, who teaches at the University of Paris 1 Panthéon-Sorbonne, faced harassment in online forums and over email. He was accused without evidence of being an agent of an invisible totalitarian world government. He was likened to criminals and dictators.

“I wasn’t a researcher anymore, I was Pol Pot, Stalin, Hitler,” Mr. Moreno said. “I have become, in one week, Public Enemy No. 1.”

For high-profile figures, such as the infectious-disease expert Dr. Anthony S. Fauci and the Microsoft co-founder Bill Gates, misinformation and the hostility it can cause have long been a part of the job description. But increasingly, even professors and researchers without much of a public persona have faced intimidation from extremists and conspiracy theorists.

Many of the recent threats have been directed at scientists studying Covid-19. In a survey of 321 such scientists who had given media interviews, the journal Nature found that 22 percent had received threats of physical or sexual violence and 15 percent had received death threats. Last year, an Austrian doctor who was a vocal supporter of vaccines and a repeated target of threats died by suicide.

One epidemiologist keeps a folder on her computer to store all the death threats she receives just “in case.” A professor of atmospheric science who studied global warming received a letter containing white powder (it looked like anthrax but turned out to be cornstarch). A professor of health law and science policy, in an article touching on his experiences with death threats, lawsuits and online trolling, wrote: “My skin is thick. I’m used to the hate.”

Mr. Moreno’s work has not been focused on the pandemic, though his 15-minute cities idea has become more popular since it began. Like many of his academic peers who have faced harassment and disinformation campaigns, he is at a loss for ways to protect himself.

“I’m not totally sure what is the best reaction — to respond, to not respond, to call a press conference, to write a press release,” he said. Academics, he said, “are relatively alone.”

Mr. Moreno, who grew up in Colombia, began working as a researcher in a computer science and robotics lab in Paris in 1983; the career that followed involved creating a start-up, meeting the Dalai Lama and being named a knight of the Légion d’Honneur. His work has won several awards and spanned many fields — automotive, medical, nuclear, military, even home goods.

Around 2010, he started thinking about how technology could help create sustainable cities. Eventually, he refined his ideas about “human smart cities” and “living cities” into his 2016 proposal for 15-minute cities. The idea owes much to its many predecessors: “neighborhood units” and “garden cities” in the early 1900s, the community-focused urban planning pioneered by the activist Jane Jacobs in the 1960s, even support for “new urbanism” and walkable cities in the 1990s. So-called low-traffic neighborhoods, or LTNs, have been set up in several British cities over the past few decades.

Critics of 15-minute cities have been outspoken, arguing that a concept developed in Europe may not translate well to highly segregated American cities. A Harvard economist wrote in a blog post for the London School of Economics and Political Science in 2021 that the concept was a “dead end” that would exacerbate “enormous inequalities in cities” by subdividing without connecting them.

Mr. Moreno did not face harassment, however, until conspiracy theorists mistakenly conflated 15-minute cities with the low-traffic-neighborhood idea in Britain.

Efforts to adopt LTNs, which were approved for testing last year in centuries-old Oxford, have drawn concerns about whether the traffic reduction measures could cause congestion to spill into surrounding areas or make some properties less accessible. Some people, however, seized on other elements of the plan — including cameras meant to monitor license plates.

The result, according to misinformed conspiracy theorists: A nightmare scenario in which residents would be confined in open-air prisons fenced off into siloed zones. On Feb. 18, when an estimated 2,000 demonstrators converged at a protest in Oxford, some carried signs claiming that 15-minute cities would become “ghettos” created by the World Economic Forum as a form of “tyrannical control.”

In fact, LTNs are championed by the Oxfordshire county council; the separate Oxford City Council has cited the 15-minute city as an inspiration for its vision of the city in 2040. As both government bodies noted in an attempt to debunk the rumors, neither proposal involves physical barriers. One concept is concerned with limiting cars, while the other is focused on bringing daily necessities closer to residents.

Still, Jordan Peterson, a Canadian psychologist with four million Twitter followers, suggested that 15-minute cities were “perhaps the worst imaginable perversion” of the idea of walkable neighborhoods. He linked to a post about the “Great Reset,” an economic recovery plan proposed by the World Economic Forum that has spawned hordes of rumors about a pandemic-fueled plot to destroy capitalism.

A member of Britain’s Parliament said that 15-minute cities were “an international socialist concept” that would “cost us our personal freedoms.” QAnon supporters said the derailment of a train carrying hazardous chemicals in Ohio was an intentional move meant to push rural residents into 15-minute cities.

“Conspiracy-mongers have built a complete story: climate denialism, Covid-19, anti-vax, 5G controlling the brains of citizens, and the 15-minute city for introducing a perimeter for day-to-day life,” Mr. Moreno said. “This storytelling is totally insane, totally irrational for us, but it makes sense for them.”

The multipronged conspiracy theory quickly became “turbocharged” after the Oxford protest, said Jennie King, head of climate research and policy at the Institute for Strategic Dialogue, a think tank that studies online platforms.

“You have this snowball effect of a policy, which in principle was only going to affect a small urban population, getting extrapolated and becoming this crucible where far-right groups, industry-sponsored lobbying groups, conspiracist movements, anti-lockdown groups and more saw an opportunity to insert their worldview into the mainstream and to piggyback on the news cycle,” she said.

The vitriol currently directed at Mr. Moreno and researchers like him mirrors “the broader erosion of trust in experts and institutions,” Ms. King said. Modern conspiracy theorists and extremists turn the people they disagree with into scapegoats for a vast array of societal ills, blaming them personally for causing the high cost of living or various health crises and creating an “us-versus-them” environment, she said.

The ramped-up rhetoric and the disintegration of safeguards has caused many people in the academic community to flee forums like Twitter for more niche sites like Mastodon, Ms. King said. Last year, the American Psychological Association published a feature suggesting that universities form safety offices to help professors filter menacing messages, scrub their personal information from the internet and gain access to counseling.

Mr. Moreno said he did not understand the intensity of the hate directed at him.

“I am not a politician, I am not a candidate for anything — as a researcher, my duty is to explore and deepen my ideas with scientific methodology,” he said. “It is totally unbelievable that we could receive a death threat just for working as scientists.”