Hackers have flooded the news feeds of thousands of Facebook users around the world with pornographic and violent images.
Users reported logging in to find themselves confronted by images and videos that had been doctored to show celebrities, including teen star Justin Bieber and singer Rihanna, in indecent poses, as well as other sexually explicit content.
Some users were tricked into sharing offensive content with others, after pasting "malicious" content into their browser URL bar.
The source of the attacks, which have been occurring over recent days, is not yet known, Facebook has said.
Initially, there was speculation that Anonymous, a loose global affiliation of hackers which has targeted other major companies in the past, could be behind the attack.
A group claiming allegiance to Anonymous claimed in a YouTube video that it had created the "Fawkes virus" to attack Facebook.
An automated voice in the video said: "Using a simple Facebook account, the worm can be carried into other accounts with little or no interaction.
"We did not expect the intensity with which this would spread."
However, Sky News understands Facebook's own investigations have now concluded that the so-called "hacktivist" group was not behind the attack.
A spokesman said in a statement: "We experienced a co-ordinated spam attack that exploited a browser vulnerability.
"Our team responded quickly and we have eliminated most of the spam caused by this attack.
£We are now working to improve our systems to better defend against similar attacks in the future.
"During this spam attack users were tricked into pasting and executing malicious javascript in their browser URL bar causing them to unknowingly share this offensive content.
£No user data or accounts were compromised."