A campaign has been launched on Twitter urging people not to share the video purporting to show the murder of James Foley.
Celebrities including actress Mia Farrow were among those who urged their followers to shun the horrific beheading footage, which the Islamic State claimed showed the death of the 40-year-old American.
Thousands of ordinary users from around the world have also backed the plan to deny IS publicity in horror at what they have done, using the hashtag #ISISmediablackout.
Twitter's chief executive Dick Costolo said the firm was taking action against accounts which spread the video.
He wrote: "We have been and are actively suspending accounts as we discover them related to this graphic imagery. Thank you."
Instead of the graphic video, large numbers of people were sharing pictures of Foley smiling at work, encouraging people to disseminate them in his memory instead.
Rosemary's Baby star Farrow tweeted: "Blackout on group that murdered James Foley. Don't share video. Give them nothing. #RespectJamesFoley."
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