Facebook said that a "coordinated spam attack" was responsible for graphic images appearing in the news feeds of some members of the world's largest social network.
Facebook, which has more than 800 million members, said some users of the social network were tricked into unknowingly sharing the offensive content.
"Recently, we experienced a coordinated spam attack that exploited a browser vulnerability," Facebook said in an email statement to AFP.
"Our efforts have drastically limited the damage caused by this attack, and we are now in the process of investigating to identify those responsible," the Palo Alto, California-based company said.
"We've built enforcement mechanisms to quickly shut down the malicious pages and accounts that attempt to exploit it," it said.
"We've put in place backend measures to reduce the rate of these attacks and will continue to iterate on our defenses to find new ways to protect people."
Facebook said protecting users from spam and malicious content "is a top priority for us and we are always working to improve our systems to isolate and remove material that violates our terms."
Facebook did not say how many members were affected by the spam but hundreds of users of the social network took to Twitter to complain about the images, which they said included hardcore pornography and graphic violence.