WASHINGTON, Aug. 15 - The Federal Bureau of Investigation has been questioning political demonstrators across the country, and in rare cases even subpoenaing them, in an aggressive effort to forestall what officials say could be violent and disruptive protests at the Republican National Convention in New York.

F.B.I. officials are urging agents to canvass their communities for information about planned disruptions aimed at the convention and other coming political events, and they say they have developed a list of people who they think may have information about possible violence. They say the inquiries, which began last month before the Democratic convention in Boston, are focused solely on possible crimes, not on dissent, at major political events.

But some people contacted by the F.B.I. say they are mystified by the bureau's interest and felt harassed by questions about their political plans.

"The message I took from it," said Sarah Bardwell, 21, an intern at a Denver antiwar group who was visited by six investigators a few weeks ago, "was that they were trying to intimidate us into not going to any protests and to let us know that, 'hey, we're watching you.' ''

The unusual initiative comes after the Justice Department, in a previously undisclosed legal opinion, gave its blessing to controversial tactics used last year by the F.B.I in urging local police departments to report suspicious activity at political and antiwar demonstrations to counterterrorism squads. The F.B.I. bulletins that relayed the request for help detailed tactics used by demonstrators - everything from violent resistance to Internet fund-raising and recruitment.

In an internal complaint, an F.B.I. employee charged that the bulletins improperly blurred the line between lawfully protected speech and illegal activity. But the Justice Department's Office of Legal Counsel, in a five-page internal analysis obtained by The New York Times, disagreed.

The office, which also made headlines in June in an opinion - since disavowed - that authorized the use of torture against terrorism suspects in some circumstances, said any First Amendment impact posed by the F.B.I.'s monitoring of the political protests was negligible and constitutional.

The opinion said: "Given the limited nature of such public monitoring, any possible 'chilling' effect caused by the bulletins would be quite minimal and substantially outweighed by the public interest in maintaining safety and order during large-scale demonstrations."

Those same concerns are now central to the vigorous efforts by the F.B.I. to identify possible disruptions by anarchists, violent demonstrators and others at the Republican National Convention, which begins Aug. 30 and is expected to draw hundreds of thousands of protesters.

In the last few weeks, beginning before the Democratic convention, F.B.I. counterterrorism agents and other federal and local officers have sought to interview dozens of people in at least six states, including past protesters and their friends and family members, about possible violence at the two conventions. In addition, three young men in Missouri said they were trailed by federal agents for several days and subpoenaed to testify before a federal grand jury last month, forcing them to cancel their trip to Boston to take part in a protest there that same day.

Interrogations have generally covered the same three questions, according to some of those questioned and their lawyers: were demonstrators planning violence or other disruptions, did they know anyone who was, and did they realize it was a crime to withhold such information.

A handful of protesters at the Boston convention were arrested but there were no major disruptions. Concerns have risen for the Republican convention, however, because of antiwar demonstrations directed at President Bush and because of New York City's global prominence.

With the F.B.I. given more authority after the Sept. 11 attacks to monitor public events, the tensions over the convention protests, coupled with the Justice Department's own legal analysis of such monitoring, reflect the fine line between protecting national security in an age of terrorism and discouraging political expression.

F.B.I. officials, mindful of the bureau's abuses in the 1960's and 1970's monitoring political dissidents like the Rev. Dr. Martin Luther King Jr., say they are confident their agents have not crossed that line in the lead-up to the conventions.

"The F.B.I. isn't in the business of chilling anyone's First Amendment rights," said Joe Parris, a bureau spokesman in Washington. "But criminal behavior isn't covered by the First Amendment. What we're concerned about are injuries to convention participants, injuries to citizens, injuries to police and first responders."

F.B.I. officials would not say how many people had been interviewed in recent weeks, how they were identified or what spurred the bureau's interest.

They said the initiative was part of a broader, nationwide effort to follow any leads pointing to possible violence or illegal disruptions in connection with the political conventions, presidential debates or the November election, which come at a time of heightened concern about a possible terrorist attack.

F.B.I. officials in Washington have urged field offices around the country in recent weeks to redouble their efforts to interview sources and gather information that might help to detect criminal plots. The only lead to emerge publicly resulted in a warning to authorities before the Boston convention that anarchists or other domestic groups might bomb news vans there. It is not clear whether there was an actual plot.

The individuals visited in recent weeks "are people that we identified that could reasonably be expected to have knowledge of such plans and plots if they existed," Mr. Parris said.

"We vetted down a list and went out and knocked on doors and had a laundry list of questions to ask about possible criminal behavior," he added. "No one was dragged from their homes and put under bright lights. The interviewees were free to talk to us or close the door in our faces."

But civil rights advocates argued that the visits amounted to harassment. They said they saw the interrogations as part of a pattern of increasingly aggressive tactics by federal investigators in combating domestic terrorism. In an episode in February in Iowa, federal prosecutors subpoenaed Drake University for records on the sponsor of a campus antiwar forum. The demand was dropped after a community outcry.

Protest leaders and civil rights advocates who have monitored the recent interrogations said they believed at least 40 or 50 people, and perhaps many more, had been contacted by federal agents about demonstration plans and possible violence surrounding the conventions and other political events.

"This kind of pressure has a real chilling effect on perfectly legitimate political activity," said Mark Silverstein, legal director for the American Civil Liberties Union of Colorado, where two groups of political activists in Denver and a third in Fort Collins were visited by the F.B.I. "People are going to be afraid to go to a demonstration or even sign a petition if they justifiably believe that will result in your having an F.B.I. file opened on you."

The issue is a particularly sensitive one in Denver, where the police agreed last year to restrictions on local intelligence-gathering operations after it was disclosed that the police had kept files on some 3,000 people and 200 groups involved in protests.

But the inquiries have stirred opposition elsewhere as well.

In New York, federal agents recently questioned a man whose neighbor reported he had made threatening comments against the president. He and a lawyer, Jeffrey Fogel, agreed to talk to the Secret Service, denying the accusation and blaming it on a feud with the neighbor. But when agents started to question the man about his political affiliations and whether he planned to attend convention protests, "that's when I said no, no, no, we're not going to answer those kinds of questions," said Mr. Fogel, who is legal director for the Center for Constitutional Rights in New York.

In the case of the three young men subpoenaed in Missouri, Denise Lieberman, legal director for the American Civil Liberties Union in St. Louis, which is representing them, said they scrapped plans to attend both the Boston and the New York conventions after they were questioned about possible violence.

The men are all in their early 20's, Ms. Lieberman said, but she would not identify them.

All three have taken part in past protests over American foreign policy and in planning meetings for convention demonstrations. She said two of them were arrested before on misdemeanor charges for what she described as minor civil disobedience at protests.

Prosecutors have now informed the men that they are targets of a domestic terrorism investigation, Ms. Lieberman said, but have not disclosed the basis for their suspicions. "They won't tell me," she said.

Federal officials in St. Louis and Washington declined to comment on the case. Ms. Lieberman insisted that the men "didn't have any plans to participate in the violence, but what's so disturbing about all this is the pre-emptive nature - stopping them from participating in a protest before anything even happened."

The three men "were really shaken and frightened by all this," she said, "and they got the message loud and clear that if you make plans to go to a protest, you could be subject to arrest or a visit from the F.B.I."