Three hundred and forty-five happy people walked off Laker Airways' DC10, the Southern Belle, this morning into the brilliant sunshine at Gatwick Airport.

Three hundred and forty-four of them were passengers who had paid $135 each to fly here on Laker's first Skytrain passenger service from New York. A normal, one-way coach fare between the two cities is $313.

The other happy person was Freddie Walker, the irrepressible chairman of the carrier, who dreamed up the idea of a no-reservations, no-frills, low-cost air service more than six years ago. He has waged a flight on both sides of the Atlantic ever since to make it a reality.

"I made $19,706 in profits today," he marvelled during a cockpit interview just before landing. "All the people we thought were there in 1971 turned up six years later."

(The Associated Press reported that Laker had no comment later in the day when the second Skytrain flight left for New York with only 111 passengers, less than the 189-seat break even point. At the London-New York fare of $103.25 each, Laker lost $8,053.50 on the trip.)

Laker said during the interview that the transatlantic shuttle service requires 65 per cent capacity on each roundtrip or 447 passengers, to break even.

The first day's flights carried 614 passengers out of a total 690 seats, or 89 per cent of capacity. All but one seat was taken on the New York to London leg.

Laker said it cost $76.64, in overhead for each one-way seat. "Once I've sold 447 seats for the flights, every seat we sell over and above that makes $113 profit for use," he said.

Although Laker personally had sold the first ticket for his new air service at 4 a.m. Monday London time, had taken the first flight to New York, drank toats with cheering throng at JFK for two hours, then boarded the first flight back to London from New York, he did not evidence the red eyes, grubbiness, and numbness so many of his passengers displayed.

"I can sloope a clothesline," he explained. He was already laying plans for his next air adventure, an inexpensive advance purchase air service to Australia from London.

The flight from Kennedy taxied away from the gate an hour and 18 minutes later, because bad weather in the New York area had delayed the arrival of the first flight from London.

Despite the lateness and the fact that some travelers had been waiting literally days to buy their tickets, it was an accommodating crowd of all ages that boarded. There appeared to be a feeling that they were being treated royally by one of England's most popular, common men.

Before boarding, the ebullient Laker paid special attention to 18-year-old Californian Nicholas Ratner. Ratner was the first to buy a ticket in New York after a 55-minute wait. After dancing in the aisle with Ratner, Laker put his arm around him and escorted him down the ramp into the plane.

After welcoming his passengers aboard, Lake gave them a running commentary from the cockpit on the commentary from the cocpit on the plane's progess from the gate through take of until it was airborne.

His announcement at 12:38 a.m. that "Your Skytrain is in the air" was greeted by applause. "Enjoy your new freedom," Laker told them. He suggested he was "an escaped jailbird" who had just pulled off the "biggest jailbreak" in history having escaped from the "Iata prison" of high air fares and taken the public with him.

"Watch out," he warned his mythical capitors. "There might be another Freddie Laker about."

Champagne was served after take-ff as the weary group settled in for the six-hour flight. Drinks were available, as were headsets for the movie. Breakfasts were served two hours before arrival to those who had purchased them when buying tickets. A continental breakfast with juice,, rolls and coffee or tea cost $1.25. A hot breakfast with an omelet and sauage was $2.00.

Despite the fact that almost every seat was filled, the flight was comfortable - as comfortable as an overnight flight can be - with plenty of leg room.

Laker said that is because he has put 35 fewer seats in then he could have. Passengers were given small replies of the Laker plane as a souvenir when boarding, and were given tote bags (for women) and ties (for men) upon arrival in London.

Laker estimated that it will take six months to see if his innovative air transportation service is a success. "Of course, I could lose money," he suggested. "The success of it depends on the people it was designed for. If people want low fares, they have to support it," he said.

There's a twinkle in his eys when he suggests the possibility that Skytrain might be a success. He admitted that he only lost money once - when he tried to build an airplane. He's never lost money on the low-cost air service he's provide. "I don't like failing you know," he said "it hurts."

He thinks there are large numbers of people - especially what he calls low-income - who could not afford to fly without his service and will be attracted to it over the next year. His identification with the service can only help bring in passengers, he believes. "I think people prefer people to machines," he said "Other airlines are impersonal things. My name on the planes personalize the airplane."

He's not afraid of competition he said. Laker praised President Carter's recent decision overturning the Civil Aeronautics Board's rejection of low fares from the IATA carrier to compete with Laker. "I'm one of his disicples," he said of Carter. "I think we have the best show in town," he said. "The other airlines only have passive imitations with restricted capacity. We have 345 seats availableeach way every day."