Fred Landis had a hard day's night ahead of him when he landed at Los Angeles International Airport on Monday evening last week.

The U.S. Customs Service takes the position that he has only himself to blame.

A psychologist and author with an anti-CIA bent, Landis says he arrived on a flight from Mexico City, only to be taken into custody, strip-searched, handcuffed to a chair, and forced to surrender the papers, videotape and microfilm in his luggage. About six hours later, he was turned over to Los Angeles police, who put him in jail and held him until midweek.

According to Landis, who was born in Chile and is an American citizen, his troubles began when his credentials were fed into a Customs Service computer and an alarming response popped up on the screen.

"It said, 'No. 1, alert interested agencies, FBI and CIA,' and some other stuff I couldn't read," asserts Landis.

A Customs Service spokesman, Lou Gerig, said "it did not say anything about the CIA" but it did produce the information that there was a warrant outstanding for Landis' arrest.

"Then they searched his bags. He only told them he'd been to Mexico, but there were articles in there indicating he'd been to Cuba," Gerig related. "They asked him, 'Why didn't you tell us you'd been to Cuba?' and he said, 'Because you didn't ask.' "

Landis, who has written a number of articles for the anti-CIA magazine, Covert Action Information Bulletin, said he had gone to Havana early last month to arrange for publication there of a book he co-authored recently on the assassination of Chilean diplomat Orlando Letelier. He said he got a visa from the Cuban Embassy in Mexico City on a separate slip of paper--instead of having it stamped in his passport--and did the same thing for a side trip to Nicaragua from Havana.

Landis maintains he did this simply to avoid hassles on returning to the United States. "In retrospect, I can see it was probably a mistake," he said.

What kept him in custody, however, in addition to some unsettled traffic citations in the state of California, was a felony warrant from Illinois charging Landis with "theft of labor or services" last year for failing to return an Avis car to the Champaign, Ill., Avis office where he had rented it last July.

Landis, who owns a house in Champaign but lives in Anaheim, Calif., says he left the car in the parking lot at the St. Louis airport in a rush to catch a plane and later tried to straighten things out by phone from the West Coast. He maintains he got the problem settled last November and that somehow the case was "reactivated."

In Champaign, however, state's attorney Tom Difanis says it was never settled to his knowledge. He said Landis was supposed to have returned the car last July 22. Difanis said he filed the theft charge last Sept. 2 and the car, a 1981 Pontiac Grand Prix, wasn't located until Sept. 7, when the St. Louis airport police found it.

Landis and Customs officials are at odds on most other details. He says he was "pushed against the wall, kicked" and shoved around. Gerig says there was no rough stuff and insists Landis was "handled in the proper way." Landis is convinced that "somebody went to an awful lot of trouble to make problems for me when I came through that airport." The spokesman at Customs maintains that "we did not have advance knowledge that he was coming into the country."

Released last Wednesday on $2,500 bond in connection with the Illinois charge, Landis protests that he has yet to get back his papers and film. He said they include his doctoral dissertation for the University of Illinois ("Psychological Warfare and Media Operations in Chile, 1970-73"), a videotape production entitled "CIA Media Operations, a Study in Imagination and Perversity," slides of the front pages of various Latin American newspapers, several issues of Covert Action Information Bulletin and other magazines, and three rolls of microfilm.

Landis said the microfilm contains 1970 editions of the Chilean newspaper, El Mercurio, that he obtained years ago from the Harvard Library.

Gerig said Customs officials "have to determine" for themselves what's on the microfilm and other materials seized from Landis under an old law prohibiting importation of "seditious or pornographic material."

" 'Seditious' is probably the area we're looking at in this case," Gerig said. If the material doesn't fit, "we'll return it to him."