From "FBI Scrutinizes Antiwar Rallies" in the New York Times:

The Federal Bureau of Investigation has collected extensive information on the tactics, training and organization of antiwar demonstrators and has advised local law enforcement officials to report any suspicious activity at protests to its counterterrorism squads, according to interviews and a confidential bureau memorandum.

The memorandum, which the bureau sent to 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.

[...]

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 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."