President Carter made his strongest stand to date in favor of "substantial deregulation" for all aspects of America's transportation industries in his Clinton, Mass., town hall question-and-answer session Wednesday night.

In a statement that caught the regulated trucking industry completely by surprise, Carter endorsed an end to "unnecessary restraints by the government bureaucracy" that require some trucks to move empty on return trips.

Answering a question from George J. Oberg, who identified himself as vice president of the New England Independent Trucking Association, Carter said his White House staff will complete "within the next few weeks" an assessment of trucking regulations by the Interstate Commerce Commission.

The President said that he already had supported Senate legislation "to deregulate to a major degree the airline industry." It was his strongest use to date of the term "deregualte," a concept which Secretary of Transportation Brock Adams repeatedly has denounced.

Carter went on to state that a message to Congress on the surface transportation industry would follow the airline proposal. "We . . .have begun to assess the need for deregulation of the transportation industry as much as possible," the President said.

"I think that in general, I and the Secretary of Transportation, Brock Adams, favor substantial deregulation of all aspects of the transportation industry and I believe that's something that you would favor," Carter added.

American Trucking Association president Bennett C. Whitlock Jr. - representing thousands of trucking firms across the nation - who launched a strong campaign to help defeat deregulation proposals of the Ford administration, said yesterday his industry "is, of course, concerned with the President's remarks . . ."

However, Whitlock continued, "we believe that the remarks will create even greater concern among the millions of small shippers and thousands of communities which have come to rely on safe, efficient regulated trucking service at reasonable, nondiscriminatory and nonprejudicial rates."

He called on Carter to give the truckers "an opportunity to be heard" before any recommendations are made to Congress.

A spokesman for Secretary Adams, who has endorsed some reforms of federal transportation regulation while opposing "deregulation," said yesterday he had "no idea" what Carter was talking about Wednesday night on the subject of motor carrier regulations.

Informed sources said Carter is expected to send a message to Congress that would not propose specific legislation but would outline goals of reduces ICC regulation over entry into the trucking business and some limitations on collective rate-setting by truckers in various regions of the country.