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In hollywood thrillers, the assassin always finds an air vent or manhole cover to crawl through to get a clear shot at his target.

But it will take more than unguarded passageways, or the fireman ruse Tom Cruise used to break into the CIA in "Mission Impossible," to breach real-life security in Washington this weekend.

As leaders from more than 40 countries gather for the 50th anniversary summit of the North Atlantic Treaty Organization, dozens of federal agencies have been marshaled to protect them in a cordon unusual even by Washington standards.

Snipers with night-vision goggles will sit on rooftops. Security units will patrol the capital's kitchens, inspecting the shallots and sides of beef. Undercover agents will wander around among the crowds, trying to blend in with power ties or Tommy Hilfiger shirts.

"We are always at a high state of alert, but we've notched things up higher," says Secret Service spokesman Jim Mackin.

Security officials say that while the event is the biggest of its kind, it is just the latest in a series of large-scale protective missions, from the Summit of the Americas to the 50th anniversary of the United Nations. Each gathering, they say, provides lessons for the next.

"After every event we do an after-action plan," scrutinizing what went right and what went wrong, Mr. Mackin says.

One lesson learned is the need to clear the summit area of "nonessential" civilians. As a result, 90,000 federal workers who would be working today can thank NATO for a three-day weekend.

The entire operation - a full year in the planning - is being run from a hidden location downtown. Streets that border a chunk of the city 13 blocks wide and 15 blocks long will be restricted to regular traffic. Nearly 11,000 officials and 3,000 press members have been credentialed, with photo IDs, to enter restricted areas.

One of the key tasks will be coordination among the Secret Service's foreign counterparts and the more than 40 US agencies involved. All three divisions of the Secret Service have swung into action with sharpshooters on rooftops and agents patrolling streets with ear pieces and Ray Bans. Other units are poring over building blueprints, searching for vulnerable points.

Security will be especially tight at the center of the gathering, the epic-sized Ronald Reagan Federal Building. "There are multiple security systems over there," points out former Secret Service agent Rich Roth, now executive director of Counter Technology, a security consulting firm in Bethesda, Md.

Security officials are also monitoring protests. A handful of permits have been issued, and scores of unlicensed protesters are expected.

Terrorism, however, is the principal concern. Serbian nationalism is at a fever pitch in the wake of NATO bombings. Agents are also well aware of a long list of international bad guys, and homegrown extremists, who might try to use the gathering to make a statement.

Another reason for vigilance is the timing of the event. "We are close to the 19th, the magic April date of the [Oklahoma City] bombing and Waco," says former agent Roth.

Besides the ubiquitous presence of uniformed officers, who this week began walking the cavernous hallways inside the Reagan building, less-visible agents will be operating throughout the city. This includes an FBI hostage rescue team, and other agents, members of the so-called back security, who will be watching to see who's watching the security details.

Perhaps the greatest concern is the threat of an unconventional attack. "The big thing now is the chem-bio area," says Bob Taubert, formerly a counterterrorism expert at the FBI, now president of The Center for Security Studies in Fredericksburg, Va. But he believes planners are aware of all potential malfeasance.

"These guys have a lot of experience in this stuff ... and should have it pretty well wired."