By Robin Respaut

(Reuters) - Electronic privacy advocates on Thursday weighed in on a high-stakes legal fight over online communications, arguing that a subpoena seeking an Occupy Wall Street protester's tweets violates his rights to free speech and privacy.

The filing from the American Civil Liberties Union, the Electronic Frontier Foundation and Public Citizen, Inc. supports Twitter's position that the individual, Occupy protestor Malcolm Harris and not Twitter itself, is the owner of the tweets and thus the proper target for any subpoena.

Manhattan Criminal Court Justice Matthew Sciarrino Jr. had earlier ruled that Harris did not have the standing to challenge the subpoena, which seeks personal information and all of Harris' tweets from September 15 through December 31, 2011.

Harris was one of 700 protesters arrested on the Brooklyn Bridge during a protest last October.

Prosecutors at the Manhattan District Attorney's office said they to needed Harris' information to determine if the tweets he posted while on the Brooklyn Bridge were "inconsistent with his anticipated trial defense" - that the police either led or escorted protesters into stepping into the roadway.

The case has caught the attention of privacy and free speech advocates, who fear that if the judge upholds the District Attorney's subpoena, it will undermine a basic tenet of Internet communications in the era of social media: that the author, and not the company whose services are used, is responsible for the content.

Thursday's brief argues that Harris should have standing to bring a First Amendment challenge because the subpoena would reveal sensitive details about him and his communications.

The motion also asserts that Harris' Fourth Amendment right against warrantless search has been violated, because the information sought by prosecutors would impair his right to move freely without government surveillance. Continued...