LONDON – Skirmishes raged across cyberspace between WikiLeaks supporters and the companies they accuse of trying to stifle the group, with websites on both sides of the battle line taken out of service or choked off by attacks.

Protestors hold signs and shout in support of WikiLeaks and its founder Julian Assange during a demonstration in front of the Federal building in New York, Dec 9, 2010. [Photo/Agencies]

The UN's top human rights official raised the alarm Thursday over officials and corporations' moves to cut off WikiLeaks' funding and starve it of server space — something she described as "potentially violating WikiLeaks' right to freedom of expression."

Navi Pillay also expressed surprise at the scale of the online attacks that have targeted major American financial players — in some cases denying access to their websites for hours at a time.

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"It's truly what media would call a cyber-war. It's just astonishing what is happening," Pillay told reporters in Geneva.

In the Netherlands, a 16-year-old boy suspected of being involved in digital attacks by Wikileaks supporters was arrested.

US Attorney General Eric Holder said the Justice Department was looking into cyber attacks on opponents of WikiLeaks and companies that have stopped doing business with it. Holder spoke at a news conference following a meeting with European Union law enforcement partners on cybersecurity, counterterrorism and data protection.

WikiLeaks has been under intense pressure since it began publishing some 250,000 secret US diplomatic cables, with attacks on its websites and threats against its founder, Julian Assange, who is now in a British jail fighting extradition to Sweden on sex crime allegations.

US officials say WikiLeaks' actions have thrown diplomacy into disarray, caused countries to curtail dealings with America and, in the case of an earlier release of classified military documents, put the lives of informants at risk.

While US allies have also criticized WikiLeaks, some world leaders have questioned the arrest of Assange.

Russian Prime Minister Vladimir Putin, questioning the reliability of leaked US cables referring to his nation as undemocratic and corrupt, said the fact that Assange is in custody shows the West has its own problems with democracy.

"Why was Mr. Assange hidden in prison?" Putin asked at a news conference. "Is this democracy?"

Brazilian President Luiz Inacio Lula da Silva said he was surprised by the lack of outcry against Assange's arrest.

"This WikiLeaks guy was arrested and I'm not seeing any protest for freedom of expression," Silva said Thursday in Brasilia. "There is nothing, nothing for freedom of expression and against the imprisonment of this guy who was doing better work than many of the ambassadors."

Many US-based Internet companies have cut their ties to WikiLeaks, including MasterCard Inc, Visa Inc, Amazon.com, PayPal Inc and EveryDNS. Those moves have hurt WikiLeaks' ability to accept donations and support publishing efforts — and touched off a bout of Web-based warfare.

Retaliatory attacks — which WikiLeaks says it does not sanction — have been claimed by a loose-knit group of "hacktivists" who gather under the handle "Anonymous."

They are using a modified version of software generally used to conduct "stress testing" on websites, according to Paul Mutton, an analyst with the London-based company Netcraft, which is tracking the attacks.

The technique allows even unsophisticated supporters to participate in attacks because all they have to do is download the file, which is then remotely operated to send a stream of bogus page requests to target websites.

Mutton said the number of computers spewing out spam had jumped from 400 to 2,000 machines on Wednesday — relatively small numbers, he said, but still apparently enough to overwhelm MasterCard and Visa.

"I've been surprised at how effective it's been," he said. "You don't need huge numbers of people to carry out an attack like that."

The surprise was shared by Internet activist Gregg Housh, who is involved with Anonymous. "I was surprised Visa and MasterCard went down," he told The Associated Press.

Housh said the number of computers at Anonymous' disposal was rising rapidly, now about 3,000 strong. But he also said supporters were running out of anti-WikiLeaks targets.

"So far today, no one has stood up and said, 'Me next,'" he said, noting that some companies threatened by online action — such as Twitter and Amazon.com, were considered too powerful to bring down.