WikiLeaks has been under intense pressure from the United States and its allies since it began posting the first of more than 250,000 U.S. State Department documents November 28.
Since then, the site has been hit with denial-of-service attacks, been kicked off servers in the United States and France, and found itself cut off from funds in the United States and Switzerland.
In response, the site has rallied supporters to mirror its content "in order to make it impossible to ever fully remove WikiLeaks from the internet," with more than 500 sites responding to the appeal by Monday evening, it said.
WikiLeaks has also posted a massive, closely encrypted file, identified as "insurance" -- a file Assange's lawyer has described as a "thermonuclear device." Assange has said the more than 100,000 people who have downloaded the file will receive the key to decoding it should anything happen to him or should the site be taken down.
"The insurance file will only be activated in the gravest of circumstances if WikiLeaks is no longer operational," Hrafnson said.
Ira Winkler, a former National Security Agency analyst, said the file is nearly impossible to decode without the key.
U.S. Attorney General Eric Holder said he has authorized "significant" actions related to a criminal investigation of WikiLeaks, saying U.S. national security has been put at risk.
"We are doing everything that we can," Holder said Monday, though he declined to answer questions about the possibility that the U.S. government could shut down WikiLeaks.
British Foreign Secretary William Hague said the leaked information is also a danger to British national security, calling the leaks "reprehensible" and "irresponsible."
"Governments have to be able to transmit confidential information, to share confidential information, of course, for them to be able to go about their job," Hague told CNN affiliate ITN. "We think it can be a danger to our national security."
Holder also refused to say whether the actions involved search warrants or requests under the Foreign Intelligence Surveillance Act, which authorizes wiretaps or other means, describing them only as "significant."
Asked Tuesday in Afghanistan for his response to the arrest, U.S. Defense Secretary Robert Gates said, "I haven't heard that, but it sounds like good news to me."