The source said the rally, which began at about 7.30 p.m. (15:30 GMT) in central Moscow, was broken up after the amount of protesters greatly exceeded the officially sanctioned number.
In early September, Russian prosecutors upheld a complaint from a religious group against the 2x2 TV company adult-oriented cartoon network for their "overt propaganda of homosexuality and pedophilia." The 2x2 company was issued with a warning and the case was referred to court.
As well as South Park, 2x2 also broadcasts The Simpsons, The Griffins, and the Cute Little Dead Girl.
The channel said that as of next week it would suspend 12 cartoon shows, including South Park and The Simpsons until the situation is resolved.
The channel faced similar accusations in March 2008, when Russian Protestant leaders submitted a request to the Prosecutor General's Office asking for the channel's license to be revoked, saying it "promotes immorality and violence."
In February 2008, Rossvyazokhrankultura, a regulatory body for television in Russia, issued warnings to the channel over the 'Happy Tree Friends' and 'The Adventures of Big Jeff' cartoon series, recommending that it cease broadcasting them to avoid legal problems.
2x2 filed on Friday a suit at an arbitration court against allegations of promoting 'extremism'.