The United States and France yesterday became involved in a dispute over air service by Pan American World Airways into Paris from London.

The problem was brewing for several days but boiled over yesterday when France refused to let all passengers get off a Pan Am plane in Paris. Pan Am returned the plane to London with its passengers on board.

On Monday, Pan Am began a flight from San Francisco to Paris that stops in London where the London-bound passengers are discharged. The Paris-bound passengers are then transferred from the Boeing 747 to a smaller 727 for the remainder of the flight. Although the French government complained that the "chance of gauge," as its called in airline parlance, was illegal, they let the plane land and the passengers get off on Monday and Tuesday.

But yesterday, the French authorities wouldn't let the passengers get off, and made Pan Am return them to London. (The French had notified Pan Am of their intentions, and the airline had advised the passengers they might not be able to get off in Paris.)

"We protested the Fench action and told them countermeasures would be considered," James Atwood, deputy assistant secretary for transportation affairs, said yesterday. Unless France accedes to the U.S. position, it is very likely that the CAB will ask Air France to submit its schedules of flights into the U.S. to the CAB, which then might cut the numbers of flights in retaliation.Any action would have to be approved by the president.

"We are in the process of seeing whether some interim accommodation with the French could be worked out to avoid action by the United States," Atwood said yesterday. "It may take a few days before we know if that's possible."

One source said the U.S. government would propose that the French government permit Pan Am to maintain its schedule into Paris using the 727 while the two governments consult.

The U.S. agrees with Pan Am's interpretation that bilateral air agreement with the British and the French allow a change of plane in London, Atwood said. Although the agreement with the French contains a provision specifying conditions under which an airline can change planes in France, the provision doesn't bar changes outside the country, he said.

The ability to change the kind of airplane is considered commerically beneficial to Pan Am - it saves fuel and helps in the positioning of aircraft - and the French believe the U.S. should pay for it. The U.S. contends it already has paid for it in the agreement with Britain.

A Pan Am spokesman said yesterday the airline would not fly the London-Paris segment of the flight today.