As the city advances its plans to expand wireless Internet service to all of Boston, some website operators and civil liberties advocates say the city-sponsored network is censoring some sites — but city officials have attributed this to glitches on the network’s filter.

On April 21, BoingBoing.net — a blog that focuses on progressive politics and online entertainment — posted on its site that Boston’s WiFi service had banned access to it for unexplained reasons. The site later posted information that stated Boston’s network filter, which is used to conform to accepted state and federal laws, banned the site because it detected a phrase that is flagged as inappropriate.

Mayor’s Office spokesman Bill Oates said BoingBoing’s complaint was the first one the city had received since the WiFi was opened across the city in October. The city has gradually been extending access to some areas, including the South End, Newbury Street and Mission Hill.

“None of these filters are perfect,” Oates said.

Although the site’s banning was inadvertent, BoingBoing co-founder Cory Doctorow said such censorship raises questions about content restrictions and what he called flawed and subjective technology often used to filter websites.

“A bunch of programmers working to express their personal prejudices through filtering patterns are not only [creating] prior restraint on speech, [but they are creating] dumb, indiscriminate, inaccurate prior restraint on speech,” he said.

“It’s a remarkably stupid idea,” he continued, “[to] hire human beings to review the whole ‘Net and accurately separate the good pages from the bad ones. Might as well count all the grains of sand on the beach using your fingers.”

Anti-censorship activist and server programmer Seth Finkelstein, who maintains a website that provides information about censorship cases, said the site was blocked as the result of an arbitrary “dumb computer program,” adding it is likely many less popular sites also get caught in Boston’s network filter.

“We only heard about this since BoingBoing is very popular,” he said. “What else was banned [but] didn’t have the ability to publicize it?”

Bennett Haselton, founder of Peacefire.org — a website that promotes freedom of speech on the Internet — said if Boston had not unblocked the website, it would have been a violation of the site operators’ rights.

“I would have supported them going to court to get the site unblocked,” he said. “It is a violation of the First Amendment for the city to block the site on their network.”

The BoingBoing example has raised the question if it is the city’s role to filter any websites at all.

Electronic Frontier Foundation spokeswoman Rebecca Jeschke said it is the city’s responsibility to provide complete access to the Internet and said it is users’ personal decision whether or not to install filters to block certain content.

“Online filters can be appropriate in your home,” she said. “Users should be able to control what they do and don’t do with their personal machines.”

Donna Rice, spokeswoman for Internet-safety advocacy group Enough is Enough, said filters are appropriate for use by public Internet providers like Boston’s, which provides access to many.

“Filtering is a less-restrictive means of guarding kids against online pornography,” Rice said. “It by no means restricts speech.”