Representatives of the Japan Automobile Manufacturers Association Inc. yesterday said American automakers' financial probloems were caused by the public's rejection of Detroit's large, gas-guzzling cars, not by imports, and warned that any restriction of Japanese imports could lead to retaliation.

Washington economist Robert Nathan testified on behalf of the JAMA before the International Trade Commission hearings on requests by the United Auto Workers union and Ford Motor Company to restrict the number of imports sold here. Nathan said import barriers would raise "a most serious problem -- the great danger of retaliation." Nathan did not elaborate.

However, during the three days of testimony so far, Speakers have given various projections on whether the Japanese would retaliate if any restrictions were imposed.

The UAW and Ford have asked the ITC to limit the number of imported cars -- especially those from Japan -- for the next five years to give American automakers time to modernize and develop their own lines of small, fuel-efficient cars to compete with foreign models.

The ITC is expected to make its recommendation to the president by Nov. 10. The president then has 60 days to make a final decision.

The UAW and Ford allege that imports were the substantial cause of injury to the domestic auto industry, resulting in 217,000 autoworkers being laid off indefinitely, 1,000 bankrupt car dealers and projected industry losses exceeding $3 billion this year.

H. William Tanaka, representing the Japanese manufacturers, said a slight rise in imported cars in the last few years did not cause Detroit's problems.

"The market for large, powerful, comfortable and gas-guzzling automobiles which had traditionally been a specialty of Detroit -- and which had produced such excellent sales and profits as late as 1978 -- suddenly collapsed," Tanaka said. "Demand shifted to fuel-efficient cars, both imported and domestically produced. One conclusion is inescapable: If Detroit had been producing in 1979 more of the type of fuel-efficient cars that it is producing today, many of the currently unemployed UAW members would still be at work and these hearings would not have been held.

"It is equally clear that, as unfortunate as the unemployment may be in human terms, it is the direct result of the American consumer's unwillingness to purchase the large cars these unemployed workers had been producing," Tanaka continued. "Restrictions on imports will restore neither that demand nor those jobs."

Representatives of Toyota Motor Sales, U.S.A., Inc., and Nissan Motor Corp. in U.S.A. last night said they expect the American auto industry to recover soon, along with the general economy, so imports will no longer be a threat. John R. Gladen, speaking for Nissan, said the new American small cars are so well made, roomy and stylish that it will be difficult for imported cars to compete with them.

The Toyota and Nissan representatives also said, in response to ITC commissioners' questions, that contrary to allegations by Ford and the UAW they have not intentionally tried to flood the U.S. market with new vehicles, although their employes work a lot of overtime.

On Thursday, the commission members quizzed Ford on why it was unprepared to compete with imported Japanese cars. Yesterday they questioned foreign car importers on why imports have increased here and whether Detroit could have accurately predicted the abrupt shift in consumers' desires for small cars last year.