A federal judge hears arguments on Tuesday on a court order directing Twitter to hand over information on the accounts of people connected to WikiLeaks in a case pitting privacy advocates against federal prosecutors.
The court order was handed down in December, but unsealed February 9 to allow Twitter to notify the users -- three supporters of the whistleblowers' website -- and give them an opportunity to appeal the decisions.
"This is an outrageous attack by the Obama administration on the privacy and free speech rights of Twitter's customers -- many of them American citizens," WikiLeaks founder Julian Assange said in a statement on Monday.
He said the order was "more shocking, at this time, (as) it amounts to an attack on the right to freedom of association, a freedom that the people of Tunisia and Egypt, for example, spurred on by the information released by WikiLeaks, have found so valuable."
The court order directed Twitter, a microblogging site, to turn over information about three three users -- Icelandic parliamentarian Birgitta Jonsdottir; Jacob Appelbaum, a US computer researcher; and Rop Gonggrijp, a Dutch volunteer for WikiLeaks.
The Electronic Frontier Foundation (EFF), the American Civil Liberties Union (ACLU) and others have challenged the order's validity, prompting Tuesday's hearing in a federal court in Alexandria, Virginia.
WikiLeaks described the hearing as "the first round in the US government's legal battle against Julian Assange."
But it said neither it nor Assange would take part in the hearing because they believed the United States lacked jurisdiction "over expressive activities beyond its borders."
However, Assange's lead council in London, Geoffrey Robertson had brought in Harvard Law Professor Alan Dershowitz as part of the legal team battling the US government's request.
Assange, meanwhile, was in London awaiting a court decision on whether he should be extradited to Sweden to face allegations of rape and sexual molestation.
The US Justice Department has been pursuing a criminal investigation of WikiLeaks, which has obtained and published hundreds of thousands of secret US military reports and diplomatic cables.
Iceland's foreign ministry last month summoned the US ambassador in Reykjavik to express "serious concern" about the court order on behalf of Jonsdottir, who distanced herself from WikiLeaks a few months ago.
WikiLeaks said Twitter was being ordered to "disclose the names, dates and locations of all persons who have used its services to receive messages from WikiLeaks or Mr Assange."
While it welcomed Twitter's response in resisting the subpoena, WikiLeaks said other service providers like Google, Facebook and Yahoo may also have received a similar demand and "may already have provided information to the government."
"We are all asking all service providers to explain whether they, too, have been served with a similar order, and whether, they have caved into it," Assange said.
The US government's bid "to obtain vast amounts of private information (could) jeopardize and chill First Amendment rights of association, of expression, of political assembly, of speech," WikiLeaks said.