The action marked the first time security at the center has been raised to that level and the first evacuation at KSC since Hurricane Floyd came close to the Cape Canaveral coast in September 1999.

"Were proceeding with an orderly closure of the Kennedy Space Center right now," KSC spokesman Joel Well said.

"Specifically that means were going to stop the operations that were doing (shuttle booster) stacking or working on the orbiters and do that safely," he said. "And once that work is completed, we will send the workers home at that point."

Wells said center managers were treating the evacuation of the center much the same as they would if a tropical storm or hurricane was bearing down on the spaceport.

A so-called "ride-out crew" of about 200 people will be working out of an emergency operations center in the KSC Launch Control Center.

Security officers also will be watching over various processing hangars and other buildings that support the nations $8 billion shuttle fleet. Round-the-clock patrols have been put in place and bus tours at the KSC Visitors Complex were cancelled.

And a helicopter was making periodic sweeps around the space centers 52-story Vehicle Assembly Building, where shuttle orbiters are mounted atop mobile launch platforms and equipped with external tanks and twin solid rocket boosters.

The spaceport started the day with normal security in place and then heightened alerts as terrorist attacks on the World Trade Center in New York and the Pentagon in Washington, D.C. unfolded. NASA officials, however, said the move was precautionary in nature.

"Im not aware of any specific threat to the Kennedy Space Center, but we are taking this threat and the activity on in New York very seriously," Wells said.

"And we want to be sure that the people here at the Kennedy Space Center are being taken care of and that were conducting all of our activities with safety as our first priority."

The action, he added, also was taken "for the security and protection of the national assets that we have here at KSC."

Also closed in the wake of the terrorist attacks: NASA headquarters in Washington, D.C., Goddard Space Flight Center in Greenbelt, Md., Johnson Space Center in Houston, Marshall Space Flight Center in Huntsville, Ala., and the Jet Propulsion Laboratory in Pasadena, Calif., among others.

Bill Berry, deputy center director at the Ames Research Center outside San Francisco said in a brief telephone interview that his center has been ordered to go to "safe and secure operations" and place their staffs on administrative leave.

Essential personnel still were on hand at each of the centers, and a flight control staff remained at NASAs Mission Control Center to work with the crew onboard the International Space Station.

Still unclear is when NASA headquarters and the agencys field centers will reopen.