Jewish and anti-racist organisations in France have threatened to sue dozens of Twitter users for posting antisemitic jokes and statements on the site using the hashtag #UnBonJuif (“a good Jew”).

Many tweets mocked the Holocaust, said “a good Jew is a dead Jew” or contained other racist statements which are illegal under French law.

The phenomenon came to media attention when Twitter said the #UnBonJuif hashtag was the third most popular in France for several hours on October 10.

Jewish students were the first to react. “The hashtag #UnBonJuif was one of the most tweeted at a time when antisemitic attacks are increasing. It led to a record number of antisemitic tweets. Twitter must take responsibility,” said a spokesman for UEJF (Union des Etudiants Juifs de France).

Jewish umbrella organisation Crif said Twitter bore some responsibility: “Twitter France must make sure that it doesn’t permit hate messages and make them accessible to the whole social media,” wrote Crif official Mark Knobel on the association’s site.

Contacted by various French media outlets, Twitter, which does not moderate its users’ messages on principle, refused to respond for several days. Technically, the company is able to block messages, but it does not do so in order to protect freedom of speech. Only three days earlier, a hashtag with three swastikas had reached the top spot on Twitter in several countries.

Antisemitic violence has grown in France by 45 per cent in the first eight months of 2012. It emerged last week that a man was beaten unconscious on his way home from a Paris synagogue during Rosh Hashanah.

Police are still investigating the alleged terror cell they dismantled last week. They revealed the group had collected details about a lawyer who has worked on Jewish-related cases, the Crif headquarters and the liberal community, whose synagogue was bombed 32 years ago.