HONG KONG — Press freedom in Hong Kong, long an enclave of liberties in the shadow of mainland China, is increasingly threatened, with journalists assaulted, news organizations censoring stories and advertisers shunning publications that rile the authorities, a new report says.

The report by the PEN American Center, a New York-based writers’ group, catalogs developments that it says amount to an alarming erosion of Hong Kong’s tradition of freewheeling news media, including self-censorship: journalists avoiding topics or skewing coverage at the behest of superiors.

“We’re ringing an early warning bell to say there are troubling signs,” Suzanne Nossel, the executive director of PEN American Center, said in an interview in Hong Kong. “When you see the pattern that comes together, it’s pretty disturbing, and there’s a sense of a deliberate hand in all of this.”

Hong Kong retained a high level of autonomy, with many rights guaranteed by law, after it returned to Chinese sovereignty from British rule in 1997. But many people in Hong Kong see Chinese political and economic power as encroaching on their autonomy. The PEN report warns that media freedom in Hong Kong has become more vulnerable since the street protests last year that exposed public discontent with the city government and the Chinese Communist Party.

“A free press is kind of an unnatural state for people who are not free,” said Mark Simon, a senior executive at Next Media Limited, whose flagship Apple Daily newspaper, which supported the pro-democracy protests, was blockaded by pro-Beijing demonstrators.

“Over all, I think, it’s not getting better, but it’s not the end of the world, either,” Mr. Simon said in an interview, referring to the state of press freedom in Hong Kong.

From late September to December, tens of thousands of Hong Kong residents joined in demonstrations that for nearly three months blocked busy roads in three areas of the city, protesting the Chinese government’s restrictive proposals for changing the electoral system here. Some local journalists complained that the police singled them out for pepper spray or rough treatment, or that opponents of the protests had turned on them.

Still, the protests highlighted the gulf between the freedoms the media has preserved in Hong Kong and the tightening restrictions in mainland China, which imprisons more reporters than any other country, according to the New York-based Committee to Protect Journalists. The protests in Hong Kong were intensely covered by the local news media, including by freelance journalists on a proliferation of blogs and websites

“There are good weeks and bad weeks at times, but the level of access the Hong Kong police give to journalists is good,” the PEN report quoted Tom Grundy, a freelance journalist and founder of the English-language website Hong Wrong, as saying. “You can get behind the police lines, and there is better access here than in Europe or North America,” Mr. Grundy said.

But the PEN report dwells on longer-term trends that it says have narrowed the range of news and views available to the city’s residents, a concern that journalists in Hong Kong have also raised. Many media owners and advertisers in Hong Kong hold big commercial stakes in mainland China and have increasingly tailored their reporting, or advertising orders, to please the authorities there, the report said.

“With most print and online media organizations in Hong Kong owned by figures with business interests in mainland China, critics argue that Hong Kong’s self-regulation has led to self-censorship in favor of those interests,” the report says.

Apart from latent pressures, there is outright intimidation. On Monday, assailants tossed a firebomb at the home of the outspoken founder of Next Media, Jimmy Lai; it was far from the first time that had happened.

Last February, the former chief editor of the Ming Pao newspaper, Kevin Lau, was grievously injured when assailants slashed him with meat cleavers. Two suspects from mainland China are scheduled to be tried in July, but no information has emerged about who ordered the attack. Ming Pao published investigative stories about Hong Kong and mainland China during Mr. Lau’s time as the top editor.

“Last year was the darkest year for Hong Kong’s press freedom since I started working in journalism back in 1981,” Sham Yee-lan, the chairwoman of the Hong Kong Journalists Association, said in an interview.

Ms. Sham said there were more than 28 attacks on journalists during last year’s street protests. She said: “Many assailants were upset by certain publications’ news coverage and tried to vent their anger at their front-line reporters.”