The Bureau of Labor Statistics, stunned by Ronald Reagan's charges that it was "jimmying" last month's producer price index, denied yesterday that it has tampered politically with any economic figure.

Janet L. Norwood, the career statistician who is commissioner of BLS, held a rare news conference to defend her agency and to explain a computational change in the September index that Reagan apparently interpreted as a political manipulation of the figures.

In the meantime, Joel Popkin, a Washington-based analyst who headed BLS' price-gathering division in the Nixon administration, led a parade of private economists who defended the agency's integrity and supported its decision to make the technical change at this time.

Knowledgeable economists in the Reagan camp either were unavailable for interviews or declined to comment on the candidate's charges.

Charls E. Walker, a key Reagan adviser who called a press conference Friday to denounce President Carter's criticism of the Federal Reserve, refused even to discuss the BLS flap. Former Treasury secretary George P. Shultz, who accompanied Walker Friday, was traveling yesterday and could not be reached.

Experts here generally were critical yesterday of Reagan's campaign charges, contending there was no real evidence to back them up and that they only undermined the integrity of the nation's economic statistics. There were similar complaints last week about Carter's comments on the Fed.

BLS has had a reputation over the past several years of remaining strictly nonpartisan and maintaining high standards of integrity. An effort by the Nixon administration in 1971 to limit the agency's independence in presenting its monthly statistical reports was exposed by the statisticians themselves.

The technical changes that Reagan described as "jimmying" involved a shift in the way BLS handles the discounts that the nation's major automakers traditionally allow local dealers as part of their efforts to clear out excess inventories at the end of each model year.

Until now, the agency simply has not accounted for these "liquidation allowances," as they are called, when it compiled the September producer price index, which covers the period when the auto companies offer these end-of-model-year discounts.

However, a year ago Norwood and other BLS officials decided -- as part of the agency's long-range proposal to improve its statistics-gathering procedures -- to begin tracking the actual discounts involved and computing them as part of the producer price index, starting this year.

Norwood said yesterday she decided to include the new figures this year, rather than putting the change off to some future time, on the belief that the public is entitled to "the most accurate data possible." Thousands of contracts hinge on changes in the price index.

To help avert any misunderstanding, BLS also published an estimate of how the price index would have performed had the change not be put into effect. The revamped index declined 0.2 percent, in its first drop in 4 1/2 years. BLS said that without the change it would have risen 0.4 percent.

Outside economists faulted BLS on two relatively minor counts: First, some suggested privately that the agency should have been more aware of the political impact that the change might have had and possibly deferred any shift until 1981.

Second, they said BLS should have adjusted the new data for seasonal patterns -- a move that most likely would have mitigated, or possibly even erased altogether, the decline in the overall index. The adjustment would be justified because the discounts come regularly every September.

The producer price report was the last scheduled to be issued before the Nov. 4 election.

BLS officials said yesterday they did not even consider the political implications of the change until the index was made public last Friday, and they did not adjust the new data for seasonal factors because they already had published the adjustments they planned for this year.

Several economists interviewed yesterday suggested that BLS may have been guilty of "excessive naivete" in not considering the political implications of an election-year change in the index, but they flatly dismissed any notion that the agency was "jimmying" the figures politically. "I just don't believe it," said Geoffrey Moore, who was commissioner of labor statistics during the Nixon administration, when BLS came under attack from the White House for refusing to politicize its monthly reports. "I have every confidence in the people that are there."

One of the few Reagan economic advisers who would comment yesterday -- Murray L. Weidenbaum, former assistant secretary of the Treasury in the Nixon administration -- said he "wasn't sure whether [the BLS action] was the greatest example of political naivete or deliberate manipulation in years.But either way, it's damned unfortunate."