Deng Yuwen, a well-known Chinese writer now living in exile near Philadelphia, has been a vocal critic of China and its authoritarian leader, Xi Jinping. Recently, China has responded aggressively, launching crude and disturbing personal attacks online.

A covert propaganda network connected to China’s security services has bombarded not only Mr. Deng but also his 16-year-old daughter with sexually suggestive and threatening posts on various social media platforms, according to researchers from Clemson University and Meta, which owns Facebook and Instagram.

The harmful content, posted by individuals using fake identities, has surfaced in replies to Mr. Deng’s posts on X, a social platform, as well as on the accounts of public schools in their community, falsely portraying his daughter as a drug user, arsonist, and prostitute.

“I tried to delete these posts,” Mr. Deng said in a Mandarin Chinese interview, “but I didn’t succeed, because today you try to delete and tomorrow they just switch to new accounts to leave attacking text and language.”

Vulgar comments targeting Mr. Deng’s daughter have also appeared on community pages on Facebook, TripAdvisor, Patch, and Niche, according to researchers.

This harassment aligns with a pattern of online intimidation that has raised concerns in Washington, Canada, and other nations where China’s aggressive tactics have escalated. The campaign involves numerous posts tied to a social media network known as Spamouflage or Dragonbridge, which is part of China’s extensive propaganda system.

China’s efforts to discredit critics, including teenagers in the U.S., represent a dangerous escalation, according to researchers.

“There’s no question that this crosses a line that they hadn’t previously crossed,” said one researcher. “I think that suggests that the lines are becoming meaningless.”

China’s propaganda apparatus has expanded its attacks on the U.S., including attempts to undermine President Biden ahead of the upcoming election.

“They’re exporting their repression efforts and human rights abuses, targeting, threatening, and harassing those who dare question their legitimacy or authority even outside China, including right here in the U.S.,” said Christopher A. Wray, director of the FBI.

Efforts to combat the harassment continue as organizations like Meta, Facebook, and others work to remove fake accounts linked to the Spamouflage network. However, the lingering impact of these attacks can persist online for years.

The cyber attacks from China pose a significant challenge for U.S. officials, with calls for a more aggressive response to protect individuals like Mr. Deng and his family.

China’s cyber harassment campaign spreads beyond its borders, targeting critics and dissidents in countries like Taiwan, Canada, and the U.S. as part of a broader effort to influence global politics.

In Mr. Deng’s case, the personal attacks intensified after he published critical articles about China’s leadership, leading to a wave of online abuse. The Spamouflage network, known for its inauthentic social media accounts and targeted harassment, continues to pose a threat to individuals like Mr. Deng and his family.

Mr. Deng, a former editor in China who now resides in the U.S., faces ongoing cyber attacks as he continues to publish critical essays and books on Chinese politics and foreign policy.

Despite the challenges, Mr. Deng remains committed to speaking out against China’s leadership and advocating for change within the Communist Party.

The cyber attacks orchestrated by China highlight the growing threats posed by state-sponsored harassment campaigns targeting individuals critical of the regime, both within China and abroad.