Flight restrictions and other strict protective measures will be imposed in New York, the Washington, D.C., area and Pennsylvania as President Bush joins tomorrow's commemorations of the one-year anniversary of the Sept. 11 attacks. But in much of the rest of the nation, officials said they do not intend to ratchet up security significantly beyond precautions already in place.

"We will be on alert, but we don't have any reason to believe we're going to have any special problems," Philadelphia Mayor John Street said Monday.

Some additional police will be on emergency standby, but most will follow usual day-to-day procedures, he said.

Officials in Chicago, Baltimore and other cities said they did not plan to post extra officers on the streets solely because of the anniversary. For months, their officers have been devoting extra attention to potential terrorist targets identified by the FBI, such as nuclear power plants, water-treatment facilities, bridges, tunnels and other places.

"It seems like such a juicy anniversary, but these terrorists play a long-term game," Baltimore Mayor Martin O'Malley said.

Since Sept. 11, law-enforcement officials have grown accustomed to intensifying security for major events, including the World Series, the Super Bowl and Independence Day celebrations. Now the places that bore the brunt of the attacks are taking similar precautions as the anniversary arrives.

Last week, the FBI issued a bulletin to law-enforcement agencies and private industries warning that authorities are investigating "a large volume of threats of undetermined reliability" but nothing specifically tied to the anniversary events. White House press secretary Ari Fleischer repeated yesterday that authorities have no credible information that terrorists are targeting memorial activities.

But he added a cautionary note: "Anniversaries can be — not necessarily always — can be occasions for heightened terrorist activity. ... Just given the fact that it's a one-year anniversary, we're going to be on our toes."

Federal officials have not raised the nation's level of alert since creating a color-coded system in March. The nation remains at the midpoint of a five-level spectrum, yellow, which signifies a significant risk of attack.

The tightest measures are planned for commemorations at the World Trade Center site, at the Pentagon and in Shanksville, Pa., the three sites where hijacked airplanes crashed Sept. 11, killing about 3,000 people.

Thousands of people are expected to gather at each location for a series of ceremonies that will be broadcast on national television. Events at the World Trade Center site and the Pentagon are open only to family members and other invited guests; the Shanksville ceremonies are open to the general public.

Attendees at all three ceremonies must pass through metal detectors and other security checkpoints, officials said. Bags will be searched, bomb-sniffing dogs will canvass the areas, and police will use devices that detect nuclear, chemical and biological weapons.

The Secret Service is coordinating security during Bush's appearances at all three sites, aided by state and local authorities.

The federal government has put temporary flight restrictions in place during all three events and recently stepped up military air patrols in Washington, D.C., New York and other cities.

U.S. military bases went on alert yesterday out of an abundance of caution and not in response to any specific threat, government officials said.

At the State Department, spokesman Richard Boucher said a cable was sent to all diplomatic posts advising them to maintain a higher state of alert tomorrow. A worldwide July 1 caution urging Americans to take extra care remains in effect.

In Washington, D.C., police are also taking extra precautions.

A U.S. Capitol Police spokesman, Lt. Dan Nichols, said his department already has extra officers at manning guard posts, patrolling in cars, and working in plainclothes.

U.S. Park Police Chief Teresa Chambers said her department will have extra officers to handle demands from numerous activities planned around the Mall.

Information from The Associated Press is included in this report.