NEW YORK — Greta Thunberg, the teenage climate activist from Sweden who delivered a much-watched speech at the United Nations on Monday (Sept 23), has been celebrated for her activism on climate change, including galvanizing the worldwide, youth-led protests of the last week. But she has also become a lightning rod, drawing attacks by television and social media commentators.

Fox News apologized after a pundit, Michael Knowles, called her “a mentally ill Swedish child” on one of its programs Monday. (Thunberg, who is 16, has Asperger’s syndrome, a form of autism, and has called it “a superpower.”) The network called the comment disgraceful, and a spokeswoman said Fox had no plans to invite Mr Knowles back.

Mr Knowles, who writes for the conservative news site The Daily Wire, defended his remarks on Twitter, where he accused Thunberg’s supporters of exploiting her. The notion that she was being used as a puppet was repeated by others, including conservative commentator Dinesh D’Souza.

The day before Thunberg’s UN speech, Mr D’Souza likened her image to ones used in Nazi propaganda, posting a photo on Twitter of Thunberg, wearing her signature long braids, next to an illustration of a young woman with a similar hairstyle standing in front of a swastika flag.

“Children — notably Nordic white girls with braids and red cheeks — were often used in Nazi propaganda,” he wrote.

“An old Goebbels technique!” he added, referring to Joseph Goebbels, the Nazi propaganda minister.

Other outspoken figures on the right sounded similar notes. Sebastian Gorka, who worked in the White House briefly under President Donald Trump, wrote on Twitter that Thunberg’s remarks were “disturbingly redolent of a victim of a Maoist ‘re-education’ camp.”

Laura Ingraham, the Fox News host, called Thunberg’s UN speech “chilling” on her Monday night show and ran a segment about how climate change “hysteria” is changing American youth.

The segment included a clip from “Children of the Corn,” the 1984 movie based on a Stephen King novel in which children in a farm town murder adults.

Thunberg is credited with helping to increase pressure on world leaders to address climate change with what began as a one-person school strike in Stockholm, and she was lauded when she testified before Congress this month. (Thunberg traveled to the United States in a solar-powered yacht from England because of the carbon emissions associated with aviation.) Former President Barack Obama called her “one of our planet’s greatest advocates” last week.

She has previously shrugged off personal attacks, characterizing them as attempts to divert attention away from the issue of climate change.

“Greta has said from the start that people attack her because they can’t attack the science; that seems undeniable,” environmentalist Bill McKibben said in an email.

“The attacks reveal the hollowness — intellectual and moral — of the climate-denying right.”

In her remarks Monday, Thunberg excoriated world leaders for a lack of urgency on the issue of climate change.

“People are dying; entire ecosystems are collapsing,” she said. “We are in the beginning of a mass extinction, and all you can talk about is money and fairy tales of eternal economic growth. How dare you!”

On Monday night, Mr Trump tweeted a clip of Thunberg’s speech, along with a message many believed to be sarcastic.

“She seems like a very happy young girl looking forward to a bright and wonderful future,” he wrote. “So nice to see!”

Thunberg, who has nearly 2 million Twitter followers of her own, clapped back in her bio, which had been changed Tuesday: “A very happy young girl looking forward to a bright and wonderful future.” THE NEW YORK TIMES