HONG KONG — Hundreds of Hong Kong police officers arrested six current or former senior staff members of an outspoken pro-democracy news website and raided the site’s headquarters on Wednesday, yet another crackdown by the government on the city’s once-vibrant independent press.

The six were arrested on suspicion of conspiring to publish seditious material, according to a statement from the police, which did not specify the news outlet. But Stand News, a seven-year-old online publication, posted brief video footage on Facebook showing police officers at the doors of one of its deputy editors, Ronson Chan, about 6 a.m. Officers then asked Mr. Chan to stop filming, claiming he was interfering with their work.

More than 200 officers entered the publication’s headquarters in Hong Kong and conducted a search, the police said. Footage and photos reviewed by The New York Times showed officers stringing orange tape across a hallway inside the office building, and apparently wheeling suitcases and boxes containing computers and other materials out of the newsroom. A photo showed at least two dozen large blue plastic boxes stacked in the building’s lobby.

Separate footage showed Patrick Lam, the acting editor in chief of Stand News, being escorted away from his home in handcuffs. A woman asked Mr. Lam if he had anything to say, to which both he and an officer replied, “Look online.” .

Denise Ho, a popular local singer who had served on the board of the news site, was also arrested, according to a post on her Facebook page.

Hong Kong officials have targeted critics across civil society, including in the news media, since the Chinese Communist Party imposed a national security law on the city in June 2020 to quell months of fierce pro-democracy protests in 2019.

Earlier this year, Apple Daily, perhaps the city’s best-known pro-democracy newspaper, was forced to close after multiple police raids of its newsroom and the arrests of several top editors and its founder, Jimmy Lai.

On Tuesday, Mr. Lai was charged with a new accusation of sedition related to the newspaper, as were six other former senior employees. Mr. Lai, one of Hong Kong’s most prominent opposition voices, had already been sentenced to 20 months in prison in relation to his support of the pro-democracy movement, and he faces up to life in prison on other charges.

Officials have sent warning letters to foreign news outlets about coverage they dislike, and several foreign journalists have been denied visas to work in the former British colony. The government has also announced plans to enact a law against so-called fake news.

After Apple Daily folded, Stand News became one of the city’s last openly pro-democracy outlets, and officials made clear that it could be targeted next.

During the 2019 protests, Stand News reporters had documented incidents including a mob attack on pro-democracy protesters in a subway station; one reporter, Gwyneth Ho, was herself attacked. (Ms. Ho, who later resigned to enter politics, is now in jail.) In June this year, Stand News said it had removed online commentaries published in May or earlier, noting that Hong Kong was beginning to target “speech crimes.”

Hong Kong’s security secretary, Chris Tang, earlier this month accused the news site of “biased, smearing and demonizing” reports about conditions at a prison in the city. Lau Siu-Kai, an adviser to Beijing, was even more blunt, telling Chinese state media that “the survival room” for opposition news outlets was shrinking.

“Stand News will come into an end,” Mr. Lau said.

It was not immediately clear whether the outlet would face charges under the national security law, which can bring up to life imprisonment. The sedition charge does not fall under the security law but rather stems from a colonial-era ordinance. But the arrests were carried out by national security police, and the warrant for the newsroom raid was also issued under the security law, the police said.

Legal experts said the arrests showed that the authorities were blurring the lines between the security law and other criminal laws in Hong Kong, essentially allowing the security law’s more sweeping provisions, such as stricter bail conditions, to be used in more cases.

The police’s national security department could “expand its authority to cover all sorts of non-NSL cases,” Thomas Kellogg, executive director of Georgetown University’s Center for Asian Law, wrote on Twitter, using an acronym for the national security law.

Mr. Chan, the Stand News editor whose home was searched on Wednesday, was not among the six arrested. He was released after being questioned and told reporters that the police had seized his laptop, phone and iPad as well as bank documents and his press accreditation.

“Stand News has always reported professionally,” Mr. Chan said. “This is obvious for the whole world to see.”

Mr. Chan also leads the Hong Kong Journalists Association, a trade organization of about 500 local journalists that has come under scrutiny.

Mr. Tang, the security secretary, accused the association in September of “infiltrating” campuses and recruiting unprofessional student journalists; he also suggested that it had received foreign funding. The security law criminalizes collusion with foreign forces.

“We’re aware of what the Hong Kong Journalists Association means to the media industry and to Hong Kong society, so we will not dissolve easily,” he said. “We will do our best to discharge our duty until the last moment.”

In a statement on Wednesday, the association said it was “deeply concerned that the police have repeatedly arrested senior members of the media and searched the offices of news organizations containing large quantities of journalistic materials within a year.”

Hong Kong officials have denied any crackdown on press freedom. In an appearance at the Foreign Correspondents Club of Hong Kong in September, Regina Ip, a pro-Beijing lawmaker, pointed to Stand News as proof that freedom of speech was intact.

“The freedom of expression is still alive and well,” she said. “Hong Kong Stand News, all these websites are still carrying on as usual.”

Joy Dong contributed research.