Thousands of new passengers have swamped the Eastern Airlines shuttle service in the last few days, producing delays of up to two hours for some passengers and evoking complaints of discomfort and inconvenience.

The sudden increase in passenger traffic on the highly popular service between New York and Washington and New York and Boston was attributed yesterday by airline spokesmen to a combination by the grounding of other airlines DC10 jumbo jets and previously planned schedule changes along the crowded routes.

Last Wednesday, the day the grouding was ordered, Eastern carried 13,000 shuttle passengers between Boston and New York and New York and Washington. The normal figure is 10,500 or 11,000.

"We're running wide open," an Eastern spokesman said. "We're pressing hard."

On Friday evening, the line of prospective passengers waiting to board the New York shuttle snaked up stairs and down corridors and hallways for hundreds of feet through the National Airport terminal.

"I've never seen one like it,' said one veteran air traveler. At least one person in the line reported a delay of two hours.

One shuttle passenger who had planned to take the 7 p.m. flight from New York to Washington on Sunday night reported finding one plane already filled and 100 persons still lined up ahead of her when she reached the gate a few minutes before 7.

The waiting passengers boarded a backup plane about 7:15 p.m. after being told the plane would be a small one and that they should check as much hand luggage as possible.

After another quarter of an hour, the passengers, seated in the hot interior of an immobile aircraft, were told luggage crews were busy elsewhere, and that the delay would continue. The plane did not leave until 8 p.m. - an hour after the passengers had expected to leave LaGuardia Airport for the one-hour trip to National Airport here.

"It's been running slow," an Eastern spokesman said. "When you're running this heavy it's impossible to turn [around] equipment that fast."

Based on available figures, last Thursday apparently was the most difficult day. On that day, Eastern added 73 shuttle flights to the 60 normally scheduled.

Thursday, the day after DC10s were ordered grounded, also was the day that American Airlines, another air carrier with a substantial Washington-LaGuardia schedule, had planned for some time to chop its service in half.

Before the DC10s were grounded, National Airlines had been flying three round-trips a day between Washington and New York. All were 727s, but on Wednesday the airline began shifting its 727s to meet needs created by the loss of DC10s on other routes.

On Wednesday, National reduced its New York-Washington schedule to a single northbound Washington-New York flight, an airline spokesman said. With 727s normally holding from 107 to 136 passengers, the sudden schedule cut took about 600 seats off the line.

The next day, Thursday, American was to put into effect its reduced summer timetable, which called for three flights between Washington and New York's LaGuardia Airport, compared with the previous six.

American's new schedule also called for a reduction from seven to four flights between New York and Boston, another route flown by the Eastern shuttle.

These changes were designed to permit American to add flights to other cities under route authority obtained by the airline this year, a spokesman said.

When the DC10s were grounded, one more flight was taken from the Washington-New York service as well as a flight between Boston and Washington and one between Boston and New York.

As with National Airlines, American used the diverted 727s to compensate on other routes for loss of DC10s.

On American Airlines the changes meant loss of about 500 seats between Washington and New York alone.

"As they began to pull equipment off and shift it to other places," an Eastern spokesman said, "there was an immediate surge of demand on ours.It's been heavey ever since."

Spokesmen for Amtrak, which operates the nation's rail passenger service, said they could not determine immediately whether they had gained any passengers in the Washington-New York-Boston corridor.

Citing a major increase in rail traffic around the nation since the onset of gasoline shortages, one Amtrak spokesman said: "we're swamped already." An official estimated that 5,000 passengers in the corridor arrive at or leave from Union Station here each day.

Donald W. Morris, Washington-Baltimore district manager for the Trialways bus line, said he had detected a slight increase in ridership in the past few days. But he suggested that some of it may be due to the start of summer vacations from schools and colleges.

A spokesman for the American Automobile Association expressed doubt that many persons had climbed on the shuttle in the past few days merely to avoid the search for gasoline on the highways.

He said adequate supplies of gas generally are available at stations on main interstate turnpikes and throughways.

As reflected in figures from the Baltimore Harbor tunnel, the number of auto trips between here and New York appeared unaffected by airline schedule changes.

Last Monday, 62,231 vehicles used the tunnel, a main north-south link. On Tuesday, the day before the DC10s were grounded, the figure was 61,156, and Wednesday it was 62,945, an increase of less than 4 percent. By Friday the figure was up to 73,895, but Fridays traditionally are busy, according to Arnold Street, a spokesman for the harbor tunnel police.