PARIS — The rumors — circulated on social media in recent weeks through worried messages, shaky videos or grainy screenshots — were alarming.

Groups of kidnappers were roaming the poorer suburbs of Paris in utility vans to snatch children. There was talk of organ trafficking. On Facebook, users posted pictures of purported abductors, urging people to be on the lookout. On Snapchat and Twitter, they posted videos of vans speeding away, proof of an attempted getaway.

Accusations focused on the Roma, sometimes called Gypsies — an estimated 15,000 to 20,000 of whom are from Eastern Europe, mainly Bulgaria and Romania, and who live in squalid camps on the fringes of France’s cities, where they face persistent discrimination, including stereotypes of rampant thievery.

It turned out that the rumors were false. No kidnappings or even attempted kidnappings have been reported to the police. But this week, the accusations were suddenly followed by an outburst of violence — the latest example of social media being used to fan prejudice — with dangerous, or even deadly, consequences.