Washington ? The government and private technology experts warned Wednesday that hackers were planning to attack thousands of Web sites Sunday in a loosely coordinated “contest” that could disrupt Internet traffic.

Organizers established a Web site, defacers-challenge.com, which was shut down early Wednesday evening. Before it was removed, the site listed in broken English the rules for hackers who might participate. It cautioned that “deface its crime” — an apparent acknowledgment that vandalizing Internet pages is illegal.

“The FBI is taking this very seriously,” FBI spokesman Bill Murray said. “Hacking is a crime, and those who participate in this activity will be investigated and brought to justice.”

The Department of Homeland Security said Wednesday it was aware of the hackers’ plans but did not expect to issue any formal public warnings. The Chief Information Officers Council, part of the Office of Management and Budget, cautioned U.S. agencies and instructed experts to tighten security at federal Web sites.

Murray said the FBI was trying to identify who set up the Web site.

Home Internet users, who typically do not operate Web sites, probably would not be affected directly, said Oliver Friedrichs, the senior manager for security response at Symantec Corp.