New York officials have said they expect hundreds of thousands of people to stage demonstrations around the convention, which begins Aug. 30.
Concern over the convention comes amid heightened security across New York over fears that foreign terrorists might strike the city again. New York remains on a "high" terrorism alert level, while most of the country is on elevated alert.
Federal investigators have infiltrated some organizations and are monitoring plans for protests being published on the Internet. The FBI also interviewed some protesters around the country before last month's Democratic convention in Boston and in anticipation of the Republican convention.
"We don't have any specific plot where we have all the variables we need to go out and take pre-emptive and judicial action," Gary Bald, assistant director of the FBI's counterterrorism division, said Wednesday.
The FBI has noted that some activist Web sites critical of Republicans and the convention include links to a site that lists Molotov cocktails, slingshots and bolt cutters as appropriate "offensive weapons" to carry during protests.
Three Democratic lawmakers on the House Judiciary Committee have criticized the FBI's interviews of protesters around the country. They asked the Justice Department's inspector general to investigate what they called "possible violations of First Amendment free speech and assembly rights."
The committee's ranking Democrat, Michigan's John Conyers, along with Reps. Robert C. "Bobby" Scott of Virginia and Jerrold Nadler of New York, said in a letter that the FBI "appears to be engaged in systematic political harassment and intimidation of legitimate anti-war protesters."
Bald, speaking with about 20 reporters at the bureau's headquarters, said FBI agents have interviewed only protesters they believed were plotting to firebomb media vehicles at the conventions or might have known about such plots. All these investigations are being treated as terrorism cases, Bald said.
"I've got more than enough to keep my people busy than to worry about if a protester is pro or con on some issue," Bald said. He called it "critically important that citizens have the ability to exercise their First Amendment rights."
Bald added, "We don't monitor rallies; we don't monitor people at rallies." But the FBI will open an investigation if it learns from tips, informants or other methods that protesters are planning to commit violent acts, Bald said.
The warning of a possible attack against media trucks, disclosed days before the Democratic convention, was based on claims by an informant who described an alleged plot by self-described anarchists in the Midwest to throw Molotov cocktails at television vans, a senior U.S. law enforcement official said.
Mark Silverstein, the Colorado legal director for the American Civil Liberties Union, said he was "very skeptical of the FBI's claim." The ACLU, which was contacted by some of the protesters who were interviewed by the FBI, said agents never asked directly about such an alleged plot.
Late Monday, the FBI issued a statement responding to a New York Times article on the intelligence gathering practices. Said assistant director Cassandra M. Chandler:
"The implication that we are conducting interviews without a legal basis is completely unfounded. The FBI is not monitoring groups, or interviewing individuals, unless we receive intelligence that such individuals or groups may be planning violent and disruptive criminal activity or have knowledge of such activity."
Meanwhile, in New York, an anti-war group planning a massive demonstration the day before the Republican convention asked a judge Wednesday to overrule city officials and let protesters gather in Central Park. Lawyers for United for Peace and Justice argued that the city's denial violates the state constitution.
Bill Dobbs, a spokesman for the group, on Wednesday night accused the FBI of "fear-mongering," which he said could keep some people from lawfully protesting at the convention.
City officials had said the expected crowd at the Aug. 29 rally, which could exceed 250,000 people, would damage the grass.
But the lawsuit noted that the park has been used in the past for such gatherings as a Paul Simon concert attended by 750,000 people, a papal Mass that drew 250,000 people and regular performances by the New York City Opera and the New York Philharmonic.
The state constitution was violated "by discriminating on the basis of content in allowing cultural but not political events," the group claims.