The new rules say FBI agents can conduct surveillance at libraries, Web sites and churches and mosques, without any evidence that a crime has been committed or is being planned, reports CBS News Correspondent Bob Fuss.
The changes were announced by Attorney General John Ashcroft and FBI Director Robert Mueller.
Ashcroft, claiming FBI agents in the field have been hampered by a range of bureaucratic restrictions, said the new guidelines would help them to do their jobs.
"These restrictions are a competitive advantage for terrorists," Ashcroft said of existing rules.
He said, for instance, that under present guidelines, FBI agents "cannot surf the Web, the way you and I can," and cannot simply walk into public events to observe people and activities.
The new guidelines give FBI agents more freedom to investigate terrorism even when they are not pursuing a particular case.
But civil liberties and rights groups warn the changes could result in a return to the days of domestic spying.
The attorney general's guidelines on surveillance were first imposed on the FBI in 1976 following disclosures that the bureau under the late J. Edgar Hoover had run a widespread domestic surveillance program called Cointelpro.
Critics said the FBI under Hoover had overstepped its authority by using Cointelpro to spy on civil rights activists including Martin Luther King and the Black Panthers, opponents of the Vietnam War and others.
"Apparently Attorney General Ashcroft wants to get the FBI back in the business of spying on religious and political organizations," said Margaret Ratner of the Center for Constitutional Rights. "That alone would be unconstitutional but history suggests the FBI won't stop at passive information gathering. We fear a return to the days of Cointelpro."
The changes in surveillance rules could also face court challenges, says CBSNews.com Legal Analyst Andrew Cohen.
"I think many of these rules will be challenged in court and rather quickly. And I think a lot of federal judges around the country will have just as much say, and maybe more, in determining how far this new surveillance goes as the Justice Department and the FBI itself," Cohen said.
President Bush endorsed the changes and said the rights of Americans won't be violated.
"We intend to honor our Constitution and respect the freedoms we hold so dear," Mr. Bush said Thursday.
"The FBI needed to change. It was an organization full of fine people who loved America but the organization didn't meet the times," he said.
Mueller said the changes "will be exceptionally helpful to us."
"Our reforms of the FBI will and must strengthen our ability to prevent future terrorist attacks," the FBI director said.
On Wednesday, Mueller announced the FBI would be changing its focus from traditional crime-fighting to combating terrorism. He also suggested for the first time that investigators might have detected the Sept. 11 terrorist plot if they had pursued leads more diligently.
The bureau has been attacked for failing to act on a memo from an agent in the Phoenix office suggesting that FBI headquarters conduct a national search of Middle Eastern men who were taking flight training in the United States prior to Sept. 11.
Mueller's acknowledgment of serious lapses prior to the attacks came amid fresh disclosures of what could be missed hints about threats from suicide hijackings, including efforts by an unidentified Middle Eastern country to buy a commercial flight simulator.
"The jury is still out on all of it," Mueller said Wednesday at FBI headquarters. "Looking at it right now, I can't say for sure it would not have, that there wasn't a possibility that we could have come across some lead that would have led us to the hijackers."