local law enforcement agencies last month in advance of antiwar demonstrations in Washington and San Francisco, detailed how protesters have sometimes used "training camps" to rehearse for demonstrations, the Internet to raise money and gas masks to defend against tear gas. The memorandum analyzed lawful activities like recruiting demonstrators, as well as illegal activities like using fake documentation to get into a secured site.

F.B.I. officials said in interviews that the intelligence-gathering effort was aimed at identifying anarchists and "extremist elements" plotting violence, not at monitoring the political speech of law-abiding protesters.

The initiative has won the support of some local police, who view it as a critical way to maintain order at large-scale demonstrations. Indeed, some law enforcement officials said they believed the F.B.I.'s approach had helped to ensure that nationwide antiwar demonstrations in recent months, drawing hundreds of thousands of protesters, remained largely free of violence and disruption.

But some civil rights advocates and legal scholars said the monitoring program could signal a return to the abuses of the 1960's and 1970's, when J. Edgar Hoover was the F.B.I. director and agents routinely spied on political protesters like the Rev. Dr. Martin Luther King Jr.

"The F.B.I. is dangerously targeting Americans who are engaged in nothing more than lawful protest and dissent," said Anthony Romero, executive director of the American Civil Liberties Union. "The line between terrorism and legitimate civil disobedience is blurred, and I have a serious concern about whether we're going back to the days of Hoover."

Herman Schwartz, a constitutional law professor at American University who has written about F.B.I. history, said collecting intelligence at demonstrations is probably legal.

But he added: "As a matter of principle, it has a very serious chilling effect on peaceful demonstration. If you go around telling people, `We're going to ferret out information on demonstrations,' that deters people. People don't want their names and pictures in F.B.I. files."

The abuses of the Hoover era, which included efforts by the F.B.I. to harass and discredit Hoover's political enemies under a program known as Cointelpro, led to tight restrictions on F.B.I. investigations of political activities.

Those restrictions were relaxed significantly last year, when Attorney General John Ashcroft issued guidelines giving agents authority to attend political rallies, mosques and any event "open to the public."

Mr. Ashcroft said the Sept. 11 attacks made it essential that the F.B.I. be allowed to investigate terrorism more aggressively. The bureau's recent strategy in policing demonstrations is an outgrowth of that policy, officials said.

"We're not concerned with individuals who are exercising their constitutional rights," one F.B.I. official said. "But it's obvious that there are individuals capable of violence at these events. We know that there are anarchists that are actively involved in trying to sabotage and commit acts of violence at these different events, and we also know that these large gatherings would be a prime target for terrorist groups."

Civil rights advocates, relying largely on anecdotal evidence, have complained for months that federal officials have surreptitiously sought to suppress the First Amendment rights of antiwar demonstrators.

Critics of the Bush administration's Iraq policy, for instance, have sued the government to learn how their names ended up on a "no fly" list used to stop suspected terrorists from boarding planes. Civil rights advocates have accused federal and local authorities in Denver and Fresno, Calif., of spying on antiwar demonstrators or infiltrating planning meetings. And the New York Police Department this year questioned many of those arrested at demonstrations about their political affiliations, before halting the practice and expunging the data in the face of public criticism.

The F.B.I. memorandum, however, appears to offer the first corroboration of a coordinated, nationwide effort to collect intelligence regarding demonstrations.

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