Worried about further terrorist attacks, federal officials shut down the nation's entire air transportation system yesterday, ordering thousands of airplanes out of the skies and prohibiting takeoffs at airports nationwide.

The unprecedented "ground stop" cleared the nation's skies of civilian aircraft for the first time in history, and poured thousands of stranded travelers onto an already clogged network of highways and rail lines.

But passenger rail and bus services shut down as well, virtually freezing public transportation in the Northeast.

Industry observers said the mess could take days to unsnarl.

The Federal Aviation Administration said it won't lift its ban on using the nation's 460 civilian airports until at least noon today.

"That's the absolute earliest that it will be lifted," said FAA spokesman Les Dorr. "The primary thing that we want to do is not have people hanging around airports. We're encouraging them to take other forms of transportation."

But few forms of transportation were spared from yesterday's scare, prompted by the airline hijackings and terrorist attacks on New York's World Trade Center and the Pentagon in northern Virginia.

Greyhound canceled indefinitely all bus service east of Cleveland and north of Richmond, Va. It also closed all bus terminals within a mile of a federal building -- 47 in all -- and suspended international border crossings into Toronto and Montreal.

MARC rail service between Washington and Baltimore was stopped, along with other commuter services from Virginia to New York. Officials said the MARC Penn line will operate on a regular schedule today, but the Brunswick and Camden lines will operate on holiday schedules.

Amtrak stopped service throughout its northeast corridor between Washington and Boston about 10:30 a.m., but resumed some trains later in the day and hoped to operate a full schedule today.

"We shut down for security reasons, but there was a level of confidence that we could return to regular service," said Rick Remington, an Amtrak spokesman.

The Port of New York and New Jersey was closed to all vessel traffic, and Coast Guard stations throughout the East, including in Baltimore, were placed on a heightened state of alert.

The FAA issued its grounding order at 9:25 a.m., after two hijacked jetliners crashed into the World Trade Center's twin towers. All aircraft in flight were ordered to land, with commercial airlines and pilots given discretion whether to proceed to their original destinations or divert to closer airports. Many chose to divert.

Mike Orloff of Potomac boarded a Southwest Airlines flight at Baltimore-Washington International Airport just after 8 a.m., heading for a business meeting in Phoenix. While the plane was in the air, the captain announced that it was landing in St. Louis instead.

"When we landed, CNN was on in the terminal and people started crying -- there were a lot of tears," Orloff said.

Like most of the thousands of passengers who were stranded, Orloff was given a voucher for another ticket. He planned to spend a day in St. Louis before seeking alternative transportation back to his home in Potomac.

Dorr, the FAA spokesman, did not know how many airplanes were in the air when the grounding order was given, but between 4,000 and 4,500 are typically aloft at any time in the United States. On a normal day, between 36,000 and 40,000 commercial aircraft will take off and land throughout the country.

The administration reported that all domestic flights were accounted for and safely on the ground by 2:07 p.m.

Yesterday's order also affected incoming international flights, some of which were diverted to airports in Canada after yesterday's attacks. At least 22 others were allowed to land in the United States, but flights that had not left the originating foreign countries were ordered not to approach American airspace.

David S. Stempler, president of the Air Travelers Association in Washington, said that even if the FAA lifts its order tomorrow, airlines will need a week to get their planes and passengers back where they belong.

Most highways in the Northeast remained open, except those near Manhattan and the Pentagon. But roads were clogged with traffic early in the day, as workers and schoolchildren headed home early and stranded commuters turned to cars.

Some roads near military installations were closed as a security precaution. David Buck, a spokesman for the Maryland State Highway Administration, said the effect was equivalent to an early rush hour or perhaps a light snowstorm.