United Airlines canceled almost 90 percent of its scheduled flights yesterday and left thousands of travelers across the nation scrambling for alternatives after the Air Line Pilots Association struck the nation's largest carrier.

Passengers holding United tickets lined up at airport television screens from Honolulu to Boston, only to see the word CANCELED appear after their flight numbers. Other airlines said they had accommodated most requests for transfers from United.

The strike is the largest labor action in the airline industry since Congress passed deregulation legislation in 1978, setting off economic reverberations that have been hard on the pocketbooks of airline employes.

The strike could tie up United for weeks as the summer travel season looms and could make discount fares harder to find because other airlines can fill seats with United passengers and cut back low fares.

United carries more than 120,000 passengers per day, or 15 percent of U.S. air travelers.

About 4,900 pilots struck, they said, because of United's insistence on establishing a lower pay scale for newly hired flight crew members. Company Chairman Richard J. Ferris has said that he must have that lower scale to remain competitive and that, if necessary, he will hire replacement pilots and slowly rebuild the schedule.

Capt. Roger Hall, chairman of United's Master Executive Council, the ALPA term for union local, said in an interview, "We think the strike is being very effective. The idea is to create an economic penalty for the company." He also said he would "like to think" the parties can return to negotiations soon.

United spokesman Chuck Novak said late yesterday that the airline operated 11 percent of its schedule, with 165 flights to 50 airports. A normal schedule is more than 1,500 flights to 149 airports.

David A. Jewell, a spokesman for the United Pilots Strike Committee, said "we don't have a major argument" with the 11 percent estimate. However, he said, several of the United flights carried employes, not revenue-paying passengers.

United made no attempt to fly nonstop transcontinental routes and eliminated all but Toronto and Tokyo from its normal list of 10 international destinations. It canceled all service at Washington Dulles Airport and Baltimore-Washington International and severely curtailed flights from Washington National. All of its domestic flights were routed through Chicago or Denver, its major U.S. hubs.

Meanwhile, American Airlines reported that its transcontinental nonstops were almost full and that its flights from Chicago, where only United is busier each day, were running 75 percent full instead of the usual 60 percent. "We set up special counters just to handle United customers," American spokesman Lowell Duncan said.

Continental Airlines, United's main competitor at Denver, reported selling more than 80 percent of its seats, compared with the usual 64 percent.

Western Airlines, which serves many of the same cities as United west of the Rockies, also reported heavy travel. A spokesman said Western is considering additional flights to Honolulu, where United normally has 50 percent of the service.

Richard Post, United's Hawaii vice president, said no problems are seen in rebooking passengers, at least through the weekend. United operated two Hawaii round trips yesterday rather than the normal 14.

Novak said United used only management pilots yesterday. Few ALPA members crossed picket lines, and United clearly is betting that that number will increase.

United also said that it did not try to use any of 500 newly trained pilots it has in waiting. The ALPA said that the 500, all unhired, are honoring picket lines.

United mechanics, represented by the International Association of Machinists and Aerospace Workers, went to work. Officials at IAM headquarters here declined to answer questions.

"There's no problem with the machinists," a senior United official said.

The Association of Flight Attendants, representing about 10,000 United employes, officially honored picket lines, but some flight attendants crossed, management and ALPA spokesmen said.

United's Ferris said that "each hour of the day we're getting stronger and better and adding more flights. We could be flying more flights -- we have enough personnel on hand to do so -- but our aim is to offer our customers a consistent, dependable product and we will add more flights only when we can offer our product consistently."

Federal mediators attempted throughout the early morning in Boston to maintain informal talks. Official negotiations broke down at 1:10 a.m. after pilots had established picket lines at many airports.

About 8:30 a.m., Helen M. Witt, chairman of the National Mediation Board, said, "We have no meetings scheduled."

Witt kept negotiators in session beyond the midnight strike deadline because, she said, "neither side was stonewalling. There was a lot of movement." She declined to be specific.

There were some indications in Boston that the ALPA negotiating team was not completely comfortable with a strike call by the Master Executive Council in Chicago. Chairman Hall, asked about that, said, "We didn't ask the negotiating committee for a recommendation" on whether to strike. "They were to report to us on where they were, and the MEC makes the decision."

Most industry observers have said that, if the strike is not quickly resolved, United and the pilots are in for a long standoff in what could be the pivotal labor-management shootout in the era of deregulated airlines.

Ferris and the pilots have been planning for months to deal with a strike. United has more than $600 million in cash on hand and a credit line exceeding $1 billion. Its average pilot is paid about $90,000 a year, United said, while the ALPA estimates the average at $80,000.

The ALPA was badly beaten by Continental Airlines, which it struck in September 1983 after Continental declared bankruptcy. The ALPA is still technically on strike, but Continental is flying more planes and making more money than ever as it moves toward reorganization. It is also paying its pilots half of what it did before the strike.

The ALPA contract ultimately signed with United will set the pattern for pending pilot talks with Delta, Trans World and Eastern airlines.

United wants a two-tier pay schedule, with the lower tier for new-hire flight crew members, who would be paid less than those currently at work for as long as 20 years. United has negotiated such contracts with machinists and flight attendants, and American Airlines has done so with its pilots.

The ALPA has permitted two-tier contracts at Piedmont and Republic airlines, but an ALPA spokesman said the two tiers become one within five years in both contracts.

Hall said, "We offered to let him Ferris set pay rates for the first five years, then let the tiers merge in six years. I just don't understand why you shut an airline down with that kind of offer on the table."

Ferris said that, "with every passing hour," more pilots and flight attendants were crossing picket lines, and he praised those "who had the personal strength and fortitude to withstand any intimidation and report to work."