Twitter is challenging a court order to turn over to law enforcement data on one of its users involved in Occupy Wall Street in a case described by a civil liberties group as a major test of online freedom of speech.

The motion filed Monday in a New York state court said the order would require Twitter to violate federal law and denies the user the ownership rights to his Twitter messages.

The case involves a Twitter user, Malcolm Harris, who is being prosecuted by for disorderly conduct in connection with the Occupy Wall Street protest on the Brooklyn Bridge last year.

The American Civil Liberties Union on Tuesday applauded Twitter's action, saying the company was standing up for free speech.

"This is a big deal," said ACLU attorney Aden Fine.

"Law enforcement agencies, both the federal government and state and city entities, are becoming increasingly aggressive in their attempts to obtain information about what people are doing on the Internet.

"And while the individual Internet users can try to defend their rights in the rare circumstances in which they find out about the requests before their information is turned over, that may not be enough."

Last month, a judge denied a motion by Harris to quash the subpoena, saying he lacked legal standing. The judge said the data stored on the Internet is not physical property and therefore does not have the same protection,

"As a user, we may think that (Internet) storage space to be like a 'virtual home,' and with that strong privacy protection similar to our physical homes," Judge Matthew Sciarrino said in his April 20 ruling.

"However, that 'home' is a block of ones and zeroes stored somewhere on someone's computer. As a consequence, some of our most private information is sent to third parties and held far away on remote network servers."

The judge also said that while a Twitter user's information and tweets contain a considerable amount of information about the user, "Twitter does not guarantee any of its users complete privacy."

Twitter responded with the motion saying its users have rights to their own tweets and other information.

"As we said in our brief, Twitter's terms of service make absolutely clear that its users 'own' their content," Twitter counsel Ben Lee said in a statement to AFP.

"Our filing with the court reaffirms our steadfast commitment to defending those rights for our users."

According to the brief, the federal Stored Communications Act and Twitter's own policies mean the users have rights to their own postings.

"To hold otherwise imposes a new and overwhelming burden on Twitter to fight for its users' rights, since the (court order) deprives its users of the ability to fight for their own rights when faced with a subpoena from New York State," the Twitter brief said.