Rant all you want in a public park. A police officer generally won't eject you for your remarks alone, however unpopular or provocative.

Say it on the Internet, and you'll find that free speech and other constitutional rights are anything but guaranteed.

Companies in charge of seemingly public spaces online wipe out content that's controversial but otherwise legal. Service providers write their own rules for users worldwide and set foreign policy when they cooperate with regimes like China. They serve as prosecutor, judge and jury in handling disputes behind closed doors.

The governmental role that companies play online is taking on greater importance as their services _ from online hangouts to virtual repositories of photos and video _ become more central to public discourse around the world. It's a fallout of the Internet's market-driven growth, but possible remedies, including government regulation, can be worse than the symptoms.

Dutch photographer Maarten Dors met the limits of free speech at Yahoo Inc.'s photo-sharing service, Flickr, when he posted an image of an early-adolescent boy with disheveled hair and a ragged T-shirt, staring blankly with a lit cigarette in his mouth.

Without prior notice, Yahoo deleted the photo on grounds it violated an unwritten ban on depicting children smoking. Dors eventually convinced a Yahoo manager that _ far from promoting smoking _ the photo had value as a statement on poverty and street life in Romania. Yet another employee deleted it again a few months later.

"I never thought of it as a photo of a smoking kid," Dors said. "It was just of a kid in Romania and how his life is. You can never make a serious documentary if you always have to think about what Flickr will delete."

There may be legitimate reasons to take action, such as to stop spam, security threats, copyright infringement and child pornography, but many cases aren't clear-cut, and balancing competing needs can get thorny.

"We often get caught in the middle between a rock and a hard place," said Christine Jones, general counsel with service provider GoDaddy.com Inc. "We're obviously sensitive to the freedoms we have, particularly in this country, to speak our mind, (yet) we want to be good corporate citizens and make the Internet a better and safer place."

In Dors' case, the law is fully with Yahoo. Its terms of service, similar to those of other service providers, gives Yahoo "sole discretion to pre-screen, refuse or remove any content." Service providers aren't required to police content, but they aren't prohibited from doing so.

While mindful of free speech and other rights, Yahoo and other companies say they must craft and enforce guidelines that go beyond legal requirements to protect their brands and foster safe, enjoyable communities _ ones where minors may be roaming.

Guidelines help "engender a positive community experience," one to which users will want to return, said Anne Toth, Yahoo's vice president for policy.

Dors ultimately got his photo restored a second time, and Yahoo has apologized, acknowledging its community managers went too far.

Heather Champ, community director for Flickr, said the company crafts policies based on feedback from users and trains employees to weigh disputes fairly and consistently, though mistakes can happen.

"We're humans," she said. "We're pretty transparent when we make mistakes. We have a record of being good about stepping up and fessing up."

But that underscores another consequence of having online commons controlled by private corporations. Rules aren't always clear, enforcement is inconsistent, and users can find content removed or accounts terminated without a hearing. Appeals are solely at the service provider's discretion.

Users get caught in the crossfire as hundreds of individual service representatives apply their own interpretations of corporate policies, sometimes imposing personal agendas or misreading guidelines.

To wit: Verizon Wireless barred an abortion-rights group from obtaining a "short code" for conducting text-messaging campaigns, while LiveJournal suspended legitimate blogs on fiction and crime victims in a crackdown on pedophilia. Two lines criticizing President Bush disappeared from AT&T Inc.'s webcast of a Pearl Jam concert. All three decisions were reversed only after senior executives intervened amid complaints.

Inconsistencies and mysteries behind decisions lead to perceptions that content is being stricken merely for being unpopular.

"As we move more of our communications into social networks, how are we limiting ourselves if we can't see alternative points of view, if we can't see the things that offend us?" asked Fred Stutzman, a University of North Carolina researcher who tracks online communities.

First Amendment protections generally do not extend to private property in the physical world, allowing a shopping mall to legally kick out a customer wearing a T-shirt with a picture of a smoking child.

With online services becoming greater conduits than shopping malls for public communications, however, some advocacy groups believe the federal government needs to guarantee open access to speech. That, of course, could also invite meddling by the government, the way broadcasters now face indecency and other restrictions that are criticized as vague.

Others believe companies shouldn't police content at all, and if they do, they should at least make clearer the rules and the mechanisms for appeal.

"Vagueness does not inspire the confidence of people and leaves room for gaming the system by outside groups," said Lauren Weinstein, a veteran computer scientist and Internet activist. "When the rules are clear and the grievance procedures are clear, then people know what they are working with and they at least have a starting point in urging changes in those rules."

But Marjorie Heins, director of the Free Expression Policy Project, questions whether the private sector is equipped to handle such matters at all. She said written rules mean little when service representatives applying them "tend to be tone-deaf. They don't see context."

At least when a court order or other governmental action is involved, "there's more of a guarantee of due process protections," said Robin Gross, executive director of the civil-liberties group IP Justice. With a private company, users' rights are limited to the service provider's contractual terms of services.

Jonathan Zittrain, a Harvard professor who recently published a book on threats to the Internet's openness, said parties unhappy with sensitive materials online are increasingly aware they can simply pressure service providers and other intermediaries.

"Going after individuals can be difficult. They can be hard to find. They can be hard to sue," Zittrain said. "Intermediaries still have a calculus where if a particular Web site is causing a lot of trouble ... it may not be worth it to them."

Unable to stop purveyors of child pornography directly, New York Attorney General Andrew Cuomo recently persuaded three major access providers to disable online newsgroups that distribute such images. But rather than cut off those specific newsgroups, all three decided to reduce administrative hassles by also disabling thousands of legitimate groups devoted to TV shows, the New York Mets and other topics.

Gordon Lyon, who runs a site that archives e-mail postings on security, found his domain name suddenly deactivated because one entry contained MySpace passwords obtained by hackers.

He said MySpace went directly to domain provider GoDaddy, which effectively shut down his entire site, rather than contact him to remove the one posting or replace passwords with asterisks. GoDaddy justified such drastic measures, saying that waiting to reach Lyon would have unnecessarily exposed MySpace passwords, including those to profiles of children.

Meanwhile, in response to complaints it would not specify, Network Solutions LLC decided to suspend a Web hosting account that Dutch filmmaker Geert Wilders was using to promote a movie that criticizes the Quran _ before the movie was even posted and without the company finding any actual violation of its rules.

Service providers say unhappy customers can always go elsewhere, but choice is often limited.

Many leading services, particularly online hangouts like Facebook and News Corp.'s MySpace or media-sharing sites such as Flickr and Google Inc.'s YouTube, have acquired a cachet that cannot be replicated. To evict a user from an online community would be like banishing that person to the outskirts of town.

Other sites "don't have the critical mass. No one would see it," said Scott Kerr, a member of the gay punk band Kids on TV, which found its profile mysteriously deleted from MySpace last year. "People know that MySpace is the biggest site that contains music."

MySpace denies engaging in any censorship and says profiles removed are generally in response to complaints of spam and other abuses. GoDaddy also defends its commitment to speech, saying account suspensions are a last resort.

Few service providers actively review content before it gets posted and usually take action only in response to complaints.

In that sense, Flickr, YouTube and other sites consider their reviews "checks and balances" against any community mob directed at unpopular speech _ YouTube has pointedly refused to delete many video clips tied to Muslim extremists, for instance, because they didn't specifically contain violence or hate speech.

Still, should these sites even make such rules? And how can they ensure the guidelines are consistently enforced?

YouTube has policies against showing people "getting hurt, attacked or humiliated," banning even clips OK for TV news shows, but how is YouTube to know whether a video clip shows real violence or actors portraying it? Either way, showing the video is legal and may provoke useful discussions on brutality.

"Balancing these interests raises very tough issues," YouTube acknowledged in a statement.

Unwilling to play the role of arbiter, the group-messaging service Twitter has resisted pressure to tighten its rules.

"What counts as name-calling? What counts as making fun of someone in a way that's good-natured?" said Jason Goldman, Twitter's director of program management. "There are sites that do employ teams of people that

do that investigation ... but we feel that's a job we wouldn't do well."

Other sites are trying to be more transparent in their decisions.

Online auctioneer eBay Inc., for instance, has elaborated on its policies over the years, to the extent that sellers can drill down to where they can ship hatching eggs (U.S. addresses only) and what items related to natural disasters are permissible (they must have "substantial social, artistic or political value"). Hypothetical examples accompany each policy.

LiveJournal has recently eased restrictions on blogging. The new harassment clause, for instance, expressly lets members state negative feelings or opinions about another, and parodies of public figures are now permitted despite a ban on impersonation. Restrictions on nudity specifically exempt non-sexualized art and breast feeding.

The site took the unusual step of soliciting community feedback and setting up an advisory board with prominent Internet scholars such as Danah Boyd and Lawrence Lessig and two user representatives elected in May.

The effort comes just a year after a crackdown on pedophilia backfired. LiveJournal suspended hundreds of blogs that dealt with child abuse and sexual violence, only to find many were actually fictional works or discussions meant to protect children. The company's chief executive issued a public apology.

Community backlash can restrain service providers, but as Internet companies continue to consolidate and Internet users spend more time using vendor-controlled platforms such as mobile devices or social-networking sites, the community's power to demand free speech and other rights diminishes.

Weinstein, the veteran computer scientist, said that as people congregate at fewer places, "if you're knocked off one of those, in a lot of ways you don't exist."