LONDON — The battle lines between supporters of the whistle-blowing Web site WikiLeaks and its detractors began to form on Sunday, as supporters erected numerous copies of the site on the Internet and the United States put pressure on Switzerland not to offer a haven to the site’s founder, Julian Assange.

Since several major Internet companies cut off services to WikiLeaks in recent days, activists have created hundreds of mirror sites, Web sites that host exact copies of another site’s content, making censorship difficult.

The collective Anonymous, an informal but notorious group of hackers and activists, also declared war on Sunday against enemies of Mr. Assange, calling on supporters to attack companies that do not support WikiLeaks and to spread the leaked material online.

Meanwhile, the American ambassador to Switzerland, Donald S. Beyer Jr., responded to signs that Mr. Assange and WikiLeaks might seek refuge in that country, warning in the weekly magazine NZZ am Sonntag that the Swiss “should very carefully consider whether to provide shelter to someone who is on the run from the law.”