About 5,000 National Guard troops were being trained and will be phased in for airport duty later this week.
At Hartsfield International Airport in Atlanta, one of the nation's two busiest airports, 102 armed National Guard personnel were expected to appear by Friday.
Carol Hallett, president of the Air Transport Association, a lobbying group for major airlines, said Americans are accepting the idea that tougher security measures will be effective. The measures include armed plainclothes air marshals on planes, stronger cockpit doors, location beacons that can't be turned off, federal security personnel, improved surveillance equipment and National Guard patrols.
"I believe that people across America believe that it is safe and secure to fly and they are ready to start flying again," said Hallett, who was attending the Travel Industry Association of America's annual marketing forum in Atlanta.
"We're very anxious to see every plane have a federal sky marshal. That really should be an immediate goal. In addition to that, the hardening of the doors is going to be essential, doors that will secure the cockpit so that the pilots cannot be ... interrupted or attacked by a hijacker or anyone else," she said.
United Airlines, the nation's second largest carrier, flew about 175,000 passengers Monday, nearly two-thirds its regular daily passenger load. "We normally fly 230,000 to 240,000 people a day so it's getting better," said spokesman Joe Hopkins.
United plans to install steel bars and locks inside the cockpit doors of its more than 500 aircraft to keep pilots safer and to reassure passengers that steps are being taken to defeat would-be hijackers. The Federal Aviation Administration approved the reinforced cockpit doors without the usual lengthy process to certify changes in aircraft that takes years.
There was limited curbside check-in at O'Hare International Airport in Chicago, but not at Midway, the city's second airport, and cars, shuttles and cabs were restricted to drop-offs with no waiting in the terminal areas.
Travelers were being urged change their habits and show up two to three hours before scheduled flights to allow time for through searches and security checks. Electronic airline tickets were being processed at check-in counters but only if a passenger could provide confirmation with a ticket number.
Ill. Gov. George Ryan and a group of business leaders took a United Airlines flight to New York City to show their faith in the improved security procedures. They will return to O'Hare aboard an American Airlines flight.
"We've got to do some extraordinary things in these extraordinary times," said Ryan, citing the economic importance of the airline industry. "We've got to get people to fly."
In Boston, about 150 National Guard troops began training to boost security at Logan International Airport and back up civilians who screen passengers and carry-on luggage.
Logan's chief of security has been criticized for embarrassing security breaches at terminal checkpoints after 10 terrorists hijacked two jetliners Sept. 11 and flew them into New York's World Trade Center and the Pentagon.
The Massachusetts Port Authority increased the number of state troopers at airline-run checkpoints after three passengers said they were able to walk through a US Airways checkpoint avoiding an X-ray machine and metal detector.
Texas Gov. Rick Perry activated more than 400 National Guard troops to improve security at the 27 commercial airports in the Lone Star State.
Ernie DeSoto, spokesman for the Houston Airport System, said things were nearly normal.
"Most of the airlines are up to 80 to 90 percent of their scheduled flights. They are almost back to normal here at Intercontinental and at Hobby," he said referring to George Bush Intercontinental Airport and William P. Hobby Airport in Houston.
Passengers were still asked to arrive two hours in advance of scheduled flight times because of the additional security. DeSoto said passengers had no complaints about the new security measures.
"They have been very agreeable to whatever they are asked," he said.
At Dallas/Fort Worth International Airport, operations were at about 79 percent of normal, according to spokeswoman Tina Sharp. There are normally 2,300 flights daily at DFW, one of the busiest airports in the nation, but most airlines, except Southwest, have cut their schedules.
American, the major airline at Miami International Airport, hoped to resume curbside check-in by the end of the day. American, Continental, United and US Airways had resumed it at Fort Lauderdale International Airport. Delta Airlines was still getting in line with new requirements.
The first 500 National Guard personnel were expected to be in place at 19 Florida airports by Friday, 100 of them at Miami. They will be assigned to curbside check-in and X-ray checkpoints.
Passengers have been moving smoothly through both airports so far this week, and the long lines were gone. Officials credited low volume for that, but the lines could grow quickly as passengers gain more confidence in flying.
Some planes flew nearly empty when most airports re-opened three days after the attacks. Frequent fliers said people at airports have been generally more observant and considerate because everyone is concerned about security.
President George W. Bush said Reagan National Airport, which is close to the White House, the Capitol, the Pentagon and other government buildings, would reopen for a limited number of flights Thursday with some of the tightest security in the nation. Federal air marshals will be on every flight into and out of Reagan National, with service initially limited to eight hub airports.
Restaurants and shops have suffered at every airport since the attacks.
Officials were concerned about security at smaller airports where some of the hijackers breezed past checkpoints on commuter flights to Boston.
"I doubt terrorists would even attempt major establishments in the near future like Dulles Airport for example, because of the newly increased security measures," said Robert Collins, president of the Aviation Crime Prevention Institute. "They would probably target general aviation planes, air charter or small regional airlines, with the hope of slipping through the cracks so to speak."
The 24-member U.S. Conference of Mayors Task Force on Airport Security Monday called for federalizing passenger, baggage and cargo security screening at all U.S. airports.