French anti-racist and Jewish organisations on Monday condemned what they called a wave of anti-Semitism sweeping over the micro-blogging site Twitter using the hashtag #unbonjuif.
The hashtag -- which in English means a good Jew -- has been one of the top trending words on French language tweets in recent days and is often followed by offensive comments such as "A good Jew can pump up your tyre with his nose".
Anti-racist groups MRAP and SOS Racisme joined the CRIF, the umbrella group representing French Jewish institutions, in expressing outrage and promising to sue people sending anti-Semitic tweets.
MRAP said in a statement that Twitter should "take the appropriate measures" to end what it called a "flood of anti-Semitism" and said it was proposing to meet with executives from the firm.
The statements came as French authorities remain on alert after a sweep this month that left one man dead and several others in detention on suspicion of being involved in the bombing of a Jewish shop or of planning other anti-Semitic attacks.
President Francois Hollande vowed after the arrests to step up security measures for the Jewish community in France, the largest in Europe.
France's SPCJ Jewish security watchdog said last week that anti-Semitic acts surged by 45 percent in the first eight months of this year and were given new impetus by deadly attacks in March by Islamic radical Mohamed Merah.
Merah went on a shooting rampage in and around the southern city of Toulouse, killing a rabbi, three Jewish children and three French paratroopers before being shot dead in a police siege.