The Maryland State Police's first line of defense against terrorism is largely invisible, tucked into the faceless sprawl of warehouses and office parks near Baltimore-Washington International Airport in a building identifiable only by the small American flag sticker affixed to a tinted glass door.

But the fledgling Homeland Security and Intelligence Bureau's calling card is everywhere. Its toll-free hotline number, flashed on electronic billboards along major highways and distributed at bridge and tunnel tollbooths, asks people to report any "suspicious activity."

Inside the bureau, beyond locked doors with signs warning officers that they "must be armed" to enter, more than 50 investigators have fielded hundreds of calls.

For the state police, which started posting the alerts last week, the request for public assistance is an attempt to cast the widest possible net for signs of terrorism.

"We're out to try and prevent the next catastrophe," said Capt. A.J. McAndrew, commander of the bureau. "Being as close as we are to D.C., we definitely have a stake in this war."

Terrorist tip lines have sprouted in many parts of the nation; Virginia began posting its own line Tuesday. Maryland's signs have evoked a mixed response. Some callers use it to vent the frustrations of a long commute, report unruly drivers or stalled cars and even ask for directions. Others, however, report behavior later deemed suspicious enough by investigators to be catalogued and probed.

Still others have responded with anger and unease, seeing the campaign as a threat to civil liberties and a state-sponsored effort to get residents to spy on one another.

"It's an overreaction that plays on our worst instincts," said state Sen. Sharon M. Grosfeld (D-Montgomery), who fought unsuccessfully last year to block legislation to broaden police powers in the name of fighting terrorism. "The state is saying, 'It's okay to feed your paranoia, your xenophobia, your hysteria.' "

The Homeland Security unit was created March 3 by Maryland's newly appointed police superintendent, Col. Edward T. Norris, to refocus the criminal investigation division and put a priority on terrorism.

"It's a new world, and I believe the police are on the front line," Norris said.

The division also produces a daily intelligence report with the latest on terrorist activity worldwide and potentially valuable local information; a recent report advised officers to be on the lookout for a tractor-trailer stolen near Baltimore. Copies are sent to the chiefs of every police agency in Maryland, to federal agencies and to the governor.

But now, with the nation on high alert, the interest is in suspicious behavior spotted by residents.

Inside the command center -- its location is undisclosed at the request of police -- maps of Maryland line the walls and officers and analysts sit at workstations creating tipsheets from information gathered from callers. Each sheet is reviewed by a supervisor, who directs information into a database and, in some cases, passes it on to undercover investigators.

Inquiries can be launched based on a wide range of clues, said Detective Sgt. Dan Cornwell, the command center's supervisor, but officers would, for example, follow up on reports of someone seen photographing a bridge, an individual lingering outside a military installation or a person idling near the Calvert Cliffs nuclear power plant.

One recent call that prompted follow-up, Cornwell said, involved someone who reported "Middle Eastern individuals" moving suitcases in and out of a house and holding large gatherings late at night.

McAndrew was quick to note that any report based on ethnicity alone would not warrant further scrutiny. But the tipline has raised concern that citizen-initiated racial profiling might send police down a dangerous path.

"The problem," said Bilal M. Ayyub, a University of Maryland professor and member of the Maryland Arab-American Committee, is that "you cannot tell the public what to be looking for on a bulletin board on the highway. People are not trained to be detectives. What constitutes suspicious behavior? Gathering? Meeting?"

McAndrew said the officers taking calls can weed out frivolous cases. "We have had tips like that, and we ask the caller, 'In addition to the person's race or ethnicity, what else is the person doing?' " But, McAndrew agreed, the bureau is entering untested crime-fighting terrain. "Our bureau is unique in that we don't deal with crime that's already occurred," he said. "Our work deals with suspicious behavior that often is not criminal."

Grosfeld said she has seen that type of crime-fighting before -- in the 1960s, when the now-notorious intelligence units operated by big-city police departments kept illegal tabs on black activists and people opposed to the Vietnam War.

"It has the risk of becoming the equivalent to McCarthyism," she said. "We're ignoring the lessons of history."

Norris disagreed, saying the scope of the threat warrants precisely this kind of crime-fighting. He said a similar effort while he was police chief in Baltimore yielded "a tremendous amount of real, important information," including leads on a man who was plotting to take down the mainframe computer at Johns Hopkins University.

Norris said he believes that the abuses of past decades led police departments to too quickly abandon their intelligence-gathering operations "on the grounds of political correctness."

"That's why we're in the mess we're in," Norris said. "They stopped gathering it altogether. In my view, we have to strike a balance."

Staff writer Leef Smith contributed to this report.

Sen. Sharon M. Grosfeld fears the hotline could become the equivalent of "McCarthyism.""It's a new world, . . . and the police are on the front line," said State Police Chief Edward T. Norris.