The letter, with 6,000 enclosed signatures, voices doubts about the latest rulings which it says have "undermined the credibility" of the International Criminal Tribunal for the Former Yugoslavia (ICTY) and requests a thorough investigation.
The associations demand that the investigating team Ban will form include representatives of victims' associations and that the findings be made public.
They say that although more than two months have passed, no official representative has refuted a letter in which Danish ICTY Judge Frederik Harhoff voiced suspicion that Meron had influenced other judges in their rulings.
Harhoff criticised the acquittals of Ante Gotovina, Mladen Markac, Momcilo Perisic, Franko Simatovic and Jovica Stanisic, suspecting that they were handed down under political pressure.
He directly accused Meron, of the United States, of pressuring ICTY judges to hand down the acquittals, saying the reason was the concern of states such as the US and Israel that a broad interpretation of superior responsibility could be dangerous for their military leaderships.