NEW YORK -- The men who showed up at John Young's door were courteous and professional. Also intimidating. They were, after all, from the FBI.
''Just a visit by the FBI has overtones," said Young, a 68-year-old activist who says the government has been monitoring a website he runs ever since the agents visited late last year. ''Whether you've done anything wrong or not, you think, 'Oh no.' "
With the Republican National Convention less than two weeks away, federal agents and city police are keeping tabs on activists and others they believe might try to cause trouble. They are making unannounced visits to people's homes, conducting interviews, and monitoring websites and meetings.
The effort has been overshadowed by far-reaching counterterrorism measures planned for the Aug. 30 to Sept. 2 convention. Officials will not discuss it in detail, other than to say investigators always act within the law.
''We're not engaging in surveillance of groups or individuals without legal predication," said Jim Margolin, a spokesman for the FBI's New York office.
''Violent acts are not protected by the US Constitution, and the FBI has a duty to prevent such acts and to identify and bring to justice those who commit them," FBI Assistant Director Cassandra M. Chandler said yesterday n a statement.
Ann Roman, a Secret Service spokeswoman, said its agents expect to respond to an increase in possible domestic threats against President Bush and other dignitaries as the convention at Madison Square Garden nears.
''How we do that specifically, I'm not going to go into," Roman said.
According to three law enforcement sources, federal agents in New York have begun interviewing people they believe might know about plots to sow mayhem at the convention, and have used surveillance against possible suspects.
The intelligence unit of the New York Police Department has been watching websites operated by self-described anarchists. It also has sent young, scruffy-looking officers posing as activists to protest-organizing meetings, said one high-ranking law enforcement source, who spoke on condition of anonymity.
Donna Lieberman, executive director of the New York Civil Liberties Union, expressed alarm, contending that few people know they have a right to turn away the FBI. ''Political interrogation without suspicion of criminal activity harkens back to the bad old days of the McCarthy era," she said. ''The FBI does not have a right to intimidate people for criticizing the government."
Officials deny the operation threatens civil rights. They note that the FBI interviews are voluntary, and that protest meetings and Internet postings being monitored are public forums.
The Justice Department, through its office of legal counsel, concluded in an April 2004 memo that two FBI bulletins were proper in alerting law enforcement officers last year about expected protests in Washington, San Francisco, and Miami.
Recent FBI bulletins about antiwar protests have urged local police to ''be alert" and report ''potentially illegal acts" to federal terrorism task forces. Illegal activity -- such as bombings, vandalism, or trespass -- ''falls outside the scope of the First Amendment," the Justice Department concluded.
In recent weeks, several people linked to anarchist groups in Colorado, Kansas, Missouri, and elsewhere have reported being ''harassed" by federal agents about the convention.
Many activists fear a repeat of the last Republican convention, in Philadelphia, where authorities were accused of rounding up protesters on trumped-up charges before they could take to the streets. Police raided a warehouse and seized puppets that protesters planned to use as props. They also arrested an organizer on misdemeanor charges and held him on $1 million bail before his case was dropped.
Authorities in New York say no preemptive strikes are planned.
''We're not looking to get people with open warrants or anything like that," the law enforcement source said. ''We'll only arrest them if they commit vandalism or other illegal acts on 'Game Day.' "