LONDON (Reuters) - Pro-and-anti Iraq war protesters have been making their point by hacking into Web sites in a display of "cyber activism," rather than with the traditional can of spray paint or placard.

Countless activists -- protesters or war hawks -- have the ability to hijack or cripple Web sites from the opposing camp, leaving in their wake a graveyard of busted and defaced links.

"This is the future of protest," said Roberto Preatoni, founder of Zone-H, an Estonian firm that monitors and records hacking attacks. Since the war in Iraq started last week, the firm has recorded over 20,000 Web site defacements.

The most notable victim was al-Jazeera, the Qatar-based satellite TV network that angered many Western television viewers earlier this week when it aired footage of dead British and American soldiers and of prisoners-of-war.

The Arab-language site, www.aljazeera.net, flickered to life Friday, but access to the English-language version remained impossible, the result of repeated hack attacks since Monday.

Thursday, visitors to the English site were greeted with a stars-and-stripes logo saying "Let Freedom Ring." Earlier Thursday, "Hacked by Patriot, Freedom Cyber Force Militia," was scrawled on the site beneath a logo containing the U.S. flag.

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Al-Jazeera was not alone. Sites on both sides of the war front have been targeted, as have sites with no obvious affiliation to the war effort.

Last week, when bombs first began to drop on Baghdad, hundreds of U.S. and British business, government and municipal Web sites were defaced with anti-war messages, security experts reported. Seemingly within hours, more hawkish hackers went on the offensive against Arab sites.

Identifying themselves with such nicknames as "Hackweiser" and "DkD," hacker and hacker groups are difficult to track down, leading victims to wonder whether the increasingly sophisticated attacks are part of a larger military arsenal. In an editorial in Friday's Guardian, Faisal Bodi, senior editor for aljazeera.net, pointed a finger at the Bush administration. "Few here doubt that the provenance of the attack is the Pentagon," he wrote.

Security experts have been quick to dismiss the existence of state-sponsored hacking initiatives. They are typically associated with private groups or individuals with a particular viewpoint to communicate -- or with the aim of gagging their opponent.

Web site defacements are often likened to digital graffiti. Being on the Web, the message tends to get wide exposure, but remains up for a short period. More worrying is when a hacker gains access to the computer server behind a Web site as it is a central repository for corporate data.

A more crude but effective attack is the so-called "denial of service" blast, when hackers blitz a site with meaningless data requests that shuts a site down completely.

Such forms of cyber activism, or Hacktivism as it's known, is not new. But with so many tit-for-tat attacks occurring online in the past week, there are renewed calls from free speech activists for a cease fire.

"People wouldn't tolerate groups that burn down book shops or news agents that sell publications they don't agree with. They shouldn't tolerate the online equivalent," said Ian Brown, director of Foundation for Information Policy Research, a British free speech thinktank.

But others are convinced the worst is yet to come. "If you take down al-Jazeera, everybody around the world knows it. And you never have to leave your house," Preatoni said.