Federal authorities will erect a security cordon around the entire Mall for Saturday's dedication of the National World War II Memorial, but officials stressed that the plans are not based on any specific threat.

Mayor Anthony A. Williams (D) said law enforcement officials have been drawing security plans for the event for a year, and federal officials encouraged people to go about their normal activities. Officials expect the Memorial Day weekend celebration to attract as many 200,000 people a day beginning Friday.

Homeland Security Department officials warned Tuesday that a string of high-profile gatherings in Washington and elsewhere could become targets for terrorists beginning this summer.

Identified as potential targets were the World War II memorial ceremony, the presidential nominating conventions in Boston and New York, the Group of Eight summit June 8-10 at Sea Island, Ga., the Summer Olympic Games in Athens and the presidential inauguration in January.

"The city itself is a target. We already know that," said D.C. Police Chief Charles H. Ramsey. "There is no specific threat that I know of. There is no specific threat that I've been told of."

For most of Saturday, federal and local authorities plan to prohibit vehicles on the roads around the Mall from the U.S. Capitol to beyond the Lincoln Memorial, including areas near the White House and Tidal Basin. Within that perimeter, a ring of barriers will be erected around the World II Memorial, at 17th Street NW between Constitution and Independence avenues, to protect attendees. Pedestrians will be channeled to entry points along a barrier wall and screened for weapons.

Crowds will be guarded by 600 officers from local and state law enforcement agencies, along with platoons of federal agents. U.S. Coast Guard and D.C. harbor police crews will guard the Potomac River. Military and civilian aviation agencies expect to intensify surveillance of the 15-mile-radius no-fly zone around the Washington Monument.

Authorities also will reroute or monitor train shipments of hazardous materials bound for the District and place federal and local emergency response teams on standby.

"It is a massive security effort to safeguard the visitors and dignitaries related to the World War II Memorial dedication. It is a template for the inauguration and the Fourth of July," said Michael A. Mason, assistant director in charge of the FBI's Washington Field Office.

U.S. Capitol Police Chief Terrance W. Gainer said: "We're asking the public to be as vigilant as I expect an officer in a police car to be. Walk with your eyes open. Don't assume innocence in activities you see. And don't assume somebody else is doing something about it."

Security officials say safeguards being announced have been planned for a year and were set in motion weeks ago.

Because vehicle access to the memorial is prohibited, the public is urged to use Metro and shuttle buses. Police will mingle with the crowds, ride buses and monitor events by closed-circuit television. Other sensor devices will be operating, and additional government aircraft will be overhead, a federal security official said.

Capitol Police have assigned officers to ride city buses in recent weeks and expanded random vehicle checks by uniformed officers.

"We're going to be doing covert activities," said Acting U.S. Park Police Chief Dwight Pettiford. "I can't get into it. We can't talk about it. Some of it is new technology. Some of it is new practices."

There will be a heavy police presence -- including regular uniformed, canine and undercover units -- during the National Symphony Orchestra's Saturday rehearsal and Sunday concert and the annual Rolling Thunder motorcycle rally, and Capitol Police will be more heavily armed than usual, officials said.

Planners concerned about World War II veterans and other elderly attendees say they are encouraged that the weather forecast calls for cooler, seasonal temperatures Saturday. Cooling tents and medical personnel on foot, bicycles and motorized carts will be available.

Although U.S. Homeland Security Secretary Tom Ridge said yesterday that there are no plans to raise the national terror threat level advisory from yellow (elevated) to orange (high), regional officials said they moved on their own to a higher alert.

In coming months, authorities are bracing for other threats by groups that may attempt to strike Washington during a time of uncertainty, said Thomas J. Lockwood, whose first day as Homeland Security's national capital region coordinator was yesterday.

"We live here in the nation's capital," said Lockwood, who will spend Saturday with District emergency management officials. "We know we have to be pretty good."

Staff writer Lori Montgomery contributed to this report.

Armed with an automatic rifle, Officer Robert Cutter stands watch outside the Capitol as authorities prepare for huge Memorial Day weekend crowds on the Mall.