Chinese computer hackers who blame the United States for the April 1 collision between an American military surveillance plane and a Chinese fighter jet launched a massive attack Monday against U.S. Web sites, including those of United Press International, the U.S. Department of Labor and the Navy's communication center.

The UPI, a Washington-based news service, said its site was breached around 3:30 a.m. Monday, when hackers replaced the site's homepage with the image of a waving Chinese national flag and anti-American slogans scrawled in a mix of Chinese characters and English letters.

The copy that appeared in English read "The Great Chinese Nation Hooray! USA Will Be With Responsibility for the Accident Totally!!! Protest USA sell Weapon to Taiwan, Break The World Peace!!! USA IS BITCH! I am from China---Peak."

Peak is the moniker for one of the hackers, the Internet security firm Vigilinx said Monday.

"No great harm seems to have been done on this occasion. It was cyber-nuisance rather than cyber-terror. But it was a warning bell that we all need to prepare better countermeasures against a growing problem," UPI Editor-in-Chief John O'Sullivan said Monday.

The hackers appeared to indiscriminately choose their target sites, hacking into the site for the California Energy Department, the site for a European scuba diving outfit, a California manufacturing firm and a non-profit organization that assists small business owners, as well as a long list of sites for American businesses and organizations.

A group known as the Hackers Union of China claimed responsibility for some of the attacks, displaying their name on the home page of the sites they infiltrated below several lines of anti-American slogans. The group replaced the home page of each site with one reading "The People's Republic of China is Everlasting. Down With USA. Down With Yankees."

On the web site www.777.wireless.com, the group posted pictures of the Chinese military and a tribute to Wang Wei, the Chinese pilot who died in the collision with the American surveillance plane. While several of the infiltrated sites were back to normal by late Monday morning, several were still displaying the Chinese hacker pages. Some sites had been completely shut down.

As of 12:30 p.m. EDT, the Navy's communication site was still down. Sites for the Labor and Health Departments were up and running.

Don Eskes, president of Filter Corp., a manufacturing company in Fresno, Calif., told Fox News that he learned his site had been hacked just after opening up Monday from one of the company's East Coast dealers. The site, which Eskes said was maintained by an independent web provider, had been repaired by 11:45 a.m. EST.

Jeff Higginbotham, network administrator for the Central Valley Business Incubator in Clovis, Calif., said his site was hacked through an attack on the outside web hosting company that maintained the advertising and promotion portion of the site. The web-based services the organization provides for its clients are hosted internally and were not affected, he said.

"It’s really just a minor inconvenience," Higginbotham told Fox News. The hacked portion of the site was still displaying the Chinese pages at noon, EDT.

Unlike recent domestic hacker assaults that crippled such giant Web companies as Yahoo! and eBay, the attacks did not infiltrate major U.S. media sites or Web companies.

According to Vigilinx, the Chinese hackers began defacing U.S. Web sites as part of a retaliation, planned to run from May 1st through May 7th, for the plane collision.

Vigilinx said reports in the "Red Guest Alliance News" said the cyber-assault, which the group is calling a "counter-attack war," was to commence at 9 a.m. EDT on April 30. The organization allegedly mobilized its force by calling a cyberspace rally around 7 a.m. EDT.

The official Xinhua News Agency has made no public comments on the so-called "hacker war," but Chinese chat rooms and unofficial media outlets were buzzing with reports of the planned attack. There were also reports of defacements of Chinese web sites, which have been attributed to American hackers.

While China's official press has not commented on the attack, Chinese cyber-surfers reportedly overwhelmingly support the "51 Hacker War." According to the results of a poll reported by China.com, nearly 85 percent of 2089 respondents firmly support the Chinese "Red Guest" action.

A partial list of hacked government sites includes those of the Navy communication station in Washington; the U.S. Mineral Resource Administrative Bureau; the U.S. Department of Labor and the U.S. Department of Health.

Last week, the National Infrastructure Protection Center issued a warning that Chinese hackers may coordinate an attack against U.S. web sites with May Day festivities Tuesday.

— The Associated Press contributed to this report