The Carter administration, facing growth opposition to legislation to implement the Panama Canal treaties, charged yesterday that congressional foes have made "highly innacurate" estimates of how much the legislation will cost U.S. taxpayers.

At a hastily called press briefing, senior state department and Pentagon officials inisted that costs to the U.S. treasury should not exceed $870 million between now and the end of the century when transfer of the canal to Panamanian control is due to be completed.

If anything, the officials added, this administration estimate is "probably on the high side" and far more in line with reality than the price tag ascribed to transfer of the canal by such opponents as Rep. George Hansen (R-Idaho). He has estimated that the transfer costs will exceed $20 billion.

The administration's rush to get its cost estimates on the record came amid signs that its request for funds and other legislation necessary to put the canal treaties into effect is facing a very rough time in Congress.

The House Merchant Marine Committee, under the leadership of its chairman, Rep. John Murphy (D-N.Y.), an outspoken opponent of surrending U.S. control of the canal, tentatively has amended the requested legislation in ways that clash directly with the provisions of the treaties.

Hopes that the administration can muster sufficient congressional backing to force Murphy into compromises on the enabling legislation were given a severe jolt Thursday when the House, by a lopsided vote of 272 to 117, eliminated $2.5 million in requested military sales credits for Panama from the fiscal 1980 military aid bill.

The administration officials, who declined to be names, sought yesterday to minimize the impact by noting that the military aid and canal legislation technically are separate issues. But the heavy vote against military assistance for Panama underscored clearly that strong anti-treaty sentiment exists in the House and that the administration doesn't have much leverage to bring against Murphy and his backers at this time.

Part of the problem stems from charges that the administration was deceptive in its statements about transfer costs at the time when the Senate was debating aproval of the treaties last year. Then, President Carter and other officials kept emphasizing that the treaty requirements for payments to Panama-about $75 million a year-would come exclusively from canal tolls and would not be charged to U.S. taxpayers.

That, administration officials insisted yesterday, is still the case. However, they added, the administration has never denied that other costs involved in the transfer-principally relocation of U.S. military facilities and special beneifts payable to canal employes-will have to be paid by the United States.

Originally the administration estimated these costs at about $350 million. Reminded of that fact yesterday, the officials said their newer estimate of $870 million is based on a better understanding of the specified problems involved.

But while conceding that the estimate doubled in a year, the officials insisted that the $870 million is an accurate projection of what the transfer costs shoule be. They denied vehemently that the figure will keep climbing into billions of dollars as opponents have charged.

In addition, they stressed, these costs should be considered in terms of the benefits that transfer of the canal allegedly will give the United States safe and quick transit for U.S. vessels, better defence of the canal and a dominant voice in the canal's management until the end of the country.