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Reddit, the popular online message forum, has long been a free-for-all for the collective id of the Internet. A few sacred commandments kept users in check: Thou shalt not spam, thou shalt not post personally identifiable information and thou shalt not break the site.

This Reddit is changing, slowly but surely, into something with a few more rules.

The company announced on Thursday that it was updating its site-wide policies to explicitly prohibit harassment against users, a move that the company said would promote free expression on Reddit without fear of retribution from a vocal minority.

“We’ve heard a lot of complaints and found that even our existing users were unhappy with the content on the site,” said Ellen Pao, chief executive of Reddit, noting a study the company conducted this year with more than 15,000 users, or so-called redditors. “We don’t think this behavior represents what Reddit is.”

From Thursday on, users who view or experience harassment on the service will be able to e-mail Reddit employees working as moderators who have the ability to remove content and ban offenders from the site. In the policy update, Reddit defines harassment as “continued actions” that would make someone “conclude that Reddit is not a safe platform to express their ideas.”

The change will certainly stoke ire among some users who, for nearly a decade, have enjoyed posting photos, comments and articles with little intervention from the company. Thousands of micro-communities around topics ranging from cooking to cute animal photos appoint their own moderators who, through the voice of the community, create rules of their own writ.

But some of these communities have become breeding grounds for hate and vitriol, which Reddit executives say is keeping other users from speaking up on the site and potentially keeping the Reddit’s current user base from expanding beyond its 200 million regular visitors.

The company said that the number one reason Reddit users do not recommend the site to others, according to the survey, is because they want to avoid exposing friends to hate and offensive content.

“Revenge porn didn’t exist in 2005. Smartphones didn’t really exist in 2005,” said Alexis Ohanian, Reddit’s executive chairman, referring to the company ban in February of posting nude photos of people without their consent. “We’re taking the standards we had 10 years ago and bringing them up to speed for 2015.”

The move also could serve to prohibit harassment that originates on Reddit and spreads to other websites, Ms. Pao said. While she declined to give examples of this behavior, others have noted that Reddit was instrumental in organizing the “GamerGate” movement, a group of video game enthusiasts — predominantly male — who were said to have systematically aimed at and harassed outspoken female critics of the video game industry across multiple sites.

What Reddit is not doing, however, is actively policing its communities. Instead, the company will rely on users to report bad actors in the community. That may not assuage concerns that the company itself is doing enough to eliminate hate speech and harassment on the platform.

Ms. Pao said the company has hired more people to work on building the overall community, some of whom will handle complaints around harassment.

In the last year, the company has made some significant changes improve its image and add rules to the site. The company now complies with requests to take down copyrighted content when applicable, and regularly posts a “transparency” report of when it has obeyed government requests to provide information on users.

The changes are not coincidental. After years of being run by only a few employees and little resources, Reddit raised a $50 million round of financing last October, backed by well-known venture capital firm Andreessen Horowitz, among a host of others. Reddit has poured resources into expanding its mobile offerings and bulking up its advertising sales organization, with the aim of Reddit eventually growing into a bigger business.

Still, critics of the site have pointed out the reticence of some brands to spend advertising dollars on Reddit, for fear of being associated with negative or inappropriate content. A friendlier image could make for an easier sell to high-dollar accounts.

Ultimately, Reddit says the policy update is about freeing up its users to speak out even more, perhaps spurring new advocates of the site who will foster even more new user growth.

“From the very start, the goal of Reddit was to get as many people expressing their ideas as freely as possible from all over the world,” Mr. Ohanian said. “My opinion on this has not wavered.”