With streets blocked off and police blimps flying overhead, rifle-toting National Guardsmen striding through Grand Central Terminal and radiation detectors in place, this city all but bristles with security and anti-terrorist armament.

Police have doubled the number of undercover officers riding the buses and subways, and video cameras provide 24-hour feeds from bridges and tunnels. The federal government has cleared a seven-mile-radius airspace "frozen zone" over Madison Square Garden -- site of the Republican National Convention -- and a high-tech, 2,000-square-foot nerve center at police headquarters will hold representatives from 66 federal, state and city law enforcement agencies.

For months, federal officials have warned of the threat of an attempted terrorist strike before the Nov. 2 presidential election, with New York City and the Republican National Convention presenting prime targets. Homeland Security Secretary Tom Ridge insists none of this should rattle New Yorkers.

"Any attempts of a terrorist will be frustrated and repelled by multiple layers of security that they will encounter all around the city, and for that matter all around the region," Ridge said last week.

New York may have never been so well guarded. But some New Yorkers find the buildup to the GOP convention unsettling. In interviews, several dozen spoke of the disruptions caused by the phalanxes of police and National Guard troops, by protesters bent on civil disobedience, and by the roving security details assigned to Republican VIPs. Many residents, particularly immigrants, worry that they will spend a week as suspects in their city.

"People are afraid now," said Mohammad Razvi, an auxiliary police officer and executive director of a respected community group that serves Pakistani and Bangladeshi immigrants. "Whenever these terror warnings go up, they are like: 'Oh, no, are they going to pick on us again?' "

About half of those interviewed in recent weeks spoke of nagging fears of terrorist attacks. Some Manhattan families have decided to leave this week; others without the resources to get away say they will avoid Lower Manhattan.

Assistant Corporation Counsel Gail Donoghue argued the city's case in court when civil libertarians challenged the Police Department's authority to conduct random searches of demonstrators' bags. But she tends to cast a jaundiced eye on official assurances of personal safety.

"If someone's willing to die, they can always pull off an attack," she said. "I have to be here, unfortunately. But I'm definitely riding my bike that week. I view the subway as an unnecessary risk."

At 90th Street and Park Avenue, Carol Kamine-Brown paused to calculate the number of blocks between Madison Square Garden and her office, in case something happens. "Well, it's about 25 blocks -- I guess that's okay," she said. "But I have no intention, none, none, none, to go anywhere near the Garden. And I plan to ride the express bus in from Brooklyn next week instead of the subway."

Transit officials say that while there may be disruptions, subways, commuter trains and buses will run on regular schedules, although buses in Midtown will face some rerouting. Typically, ridership drops about 10 percent in the week before Labor Day, with many New Yorkers on vacation. Penn Station, which lies beneath Madison Square Garden, will be open, although only one of the six exits will remain open for commuters.

The streets immediately around the Garden will be closed frequently, particularly while the convention is in evening session. City officials say trucks serving the garment and flower districts will be allowed to pass, albeit at odd hours. Area business owners have, by and large, spoken of security as a necessary intrusion and have resigned themselves to long delays.

All this security comes at a high cost. Mayor Michael Bloomberg has acknowledged that the city will spend $65 million on security, about double his original estimate. And the city comptroller, William C. Thompson Jr., has estimated that disruptions to business and traffic and transit delays could push the total city tab to more than $300 million.

City officials have talked of security as a straightforward matter, suggesting that all New Yorkers should endorse the show of force during uncertain times. Bloomberg -- who has faced criticism for lobbying so hard to bring the convention to New York -- insisted that any disruption would be minor.

"The measures are going to strike the right balance between providing security without inconveniencing New Yorkers," he said. "New York City is being well protected on land, at sea and in the air."

But antiwar activists contend that such extensive security precautions carry a downside, perhaps scaring off some of the hundreds of thousands of potential protesters.

In these final days before the convention, the police have talked in nearly equal terms of preparing for terrorists and possibly violent protesters. They have thrown up a fenced outdoor pen on the far West Side of Manhattan to hold any overflow of people arrested in demonstrations.

The New York Daily News reported this week that unnamed "police intelligence sources" had warned that "50 of the country's leading anarchists" are coming to New York and that some had "histories of violent and disruptive tactics." The newspaper ran a fuzzy photo of one such man on its front cover, treatment usually reserved for al Qaeda suspects.

The man, Richard Picariello, is an antiwar organizer but is not affiliated with any anarchist group. Nor has any public official presented any evidence that anarchists plan more than sustained civil disobedience.

Civil disobedience and direct action are tactics planned by various other groups. Several protesters rappelled down the side of the Plaza Hotel on Friday and hung a large anti-Bush banner before they were arrested by police.

Police officials have issued permits for marches and demonstrations in every corner of Manhattan. But organizers worry that if there is trouble, police may not discriminate between the violent few and the many thousands engaged in peaceful protest.

"We see a lot of misinformation that's building up the hysteria," said Jamie Moran, a member of RNC Not Welcome, a collective that includes anarchists. "The police and some in the news media are attaching terrorist labels to us . . . making it look as if the anarchists are an organized crime syndicate."

In Brooklyn, along the stretch of grocery stores, Urdu video stores, sari shops and mosques known as Little Pakistan, the ramped-up security and presence of thousands of federal law enforcement officials evokes mixed emotions in many residents.

Two members of this Pakistani community died in the Sept. 11, 2001 attacks on the World Trade Center, so many share the fear of another attack. But federal immigration and FBI agents also interviewed and temporarily detained many members of this community in the months after those attacks. Hundreds were later deported for immigration violations. As a result, there is a lingering and palpable wariness about the enforcement power of a government on full alert.

At the recent Pakistan Day parade, organizers say that so many police lined the streets that some Pakistani Americans stayed away.

"People come into this office wanting to know whether they should come out" of their homes, said Razvi, the executive director of the Council of People's Organizations, which has an office on Coney Island Avenue. "I tell people to have their identification . . . but they are likely not going into the city next week."

Emergency services officers sample the air outside Madison Square Garden as part of the security precautions for the Republican National Convention.