Over the past several days, the anti-secrecy Web site WikiLeaks has been hit with a series of blows that have seemed to threaten its survival. Its primary Web address was deactivated, its PayPal account was frozen, and its Internet server gave it the boot.

The result: WikiLeaks is now stronger than ever, at least as measured by its ability to publish online.

Blocked from using one Internet host, WikiLeaks simply jumped to another. Meanwhile, the number of "mirror" Web sites - effectively clones of WikiLeaks' main contents pages - grew from a few dozen last week to 200 by Sunday. By early Wednesday, the number of such sites surpassed 1,000.

At the same time, WikiLeaks' supporters have apparently gone on the offensive, staging retaliatory attacks against Internet companies that have cut ties to the group amid fears they could be associated with it. On Wednesday, hackers briefly shut down access to the Web sites for MasterCard and Visa, both of which had announced they had stopped processing donations to WikiLeaks.

WikiLeaks' long-term survival depends on a number of unknowns, including the fate of its principal founder, Julian Assange, who is being held in Britain while awaiting possible extradition to Sweden related to sexual-assault allegations. But the Web site's resilience in the face of repeated setbacks has underscored a lesson already absorbed by more repressive governments that have tried to control the Internet: It is nearly impossible to do.

Experts, including some of the modern online world's chief architects, say the very design of the Web makes it difficult for WikiLeaks' opponents to shut it down for more than a few hours.

"The Internet is an extremely open system with very low barriers to access and use," said Vint Cerf, Google's vice president and the co-author of the TCP/IP system, the basic language of computer-to-computer communication over the Internet. "The ease of moving digital information around makes it very difficult to suppress once it is accessible."

Thus, despite the global uproar over the release of sensitive U.S. diplomatic cables, Assange's Web site remained defiantly intact Wednesday. Over the past week it has continued to publish a steady stream of leaked State Department documents with little visible evidence of injury from repeated, anonymous cyber-attacks or the multiple attempts to cut off its access to funding and Web resources.

By contrast, companies that have pulled the plug on WikiLeaks have suffered publicly, with cyber-attacks rendering their Web sites inaccessible or slow for hours at a time.

While a group of "hacktivists" targeted MasterCard and Visa - part of "Operation Payback," they called it - anonymous assailants have also in recent days attacked PayPal, which severed relations with WikiLeaks citing violations of its terms of service.

Web sites for Swedish prosecutors and a Swedish lawyer have also been hit, as has the banking arm of the Swiss postal service, which said it had frozen Assange's account, and even the Web site of former Alaska governor Sarah Palin.

WikiLeaks' seeming invulnerability is seen by experts as a demonstration of the power of new Web-based media to take on not only governments but also the traditional news media.