President Carter met yesterday with Panama's military ruler, Gen. Omar Torrijos, to brief him on preparations for tonight's historic signing of the Panama Canal treaties.

The 70-minute meeting with Torrijos was the first of five separate, private sessions held by Carter yesterday with visiting Latin American presidents. The officials were among the first arrivals in the parade of 24 hemispheric leaders gathering here for the signing by Carter and Torrijos at the Pan American Union, headquarters of the Organization of American States.

The treaties, which would turn control of the canal over to Panama at the end of the century, represent what is perhaps the most controversial foreign policy achievement of the seven-month-old Carter administration.

The texts of the two treaties - one spelling out details of the transfer and the other guaranteeing the canal's future neutrality - were made public yesterday following their initialing by U S and Panamamian officials at the State Department.

But, before they can be put into effect, the Carter administration faces a tough and lengthy battle against powerful political forces determined to block their approval by the Senate. The opponents contend that the canal should be kept under U S control because of its importance to national security and a symbol of U.S. global prestige.

By turning the signing ceremony into a hemispheric summit meeting, the administration is attempting in large measure, to strengthen the president's hand by demonstrating to wavering senators the solid support that the treaties have throughout Latin American.

But, administration sources say, Carter also views the presence here of so many hemispheric leaders as an important opportunity to discuss with them a wide range of world issues and problems affecting their relations with the United States.

That process began yesterday when Carter followed his meeting with Torrijos by conferring separately with Peruvian President Francisco Morales Bermudez. Paraguavan President Alfredo Stroessner, Columbian President Alfronso Lopez Michelsen, Chilean President Augusto Pizochet Ugarte and Brazil's vice president, Adalberto Pereira Dos Santos.

In a brief exchange with reporters afterward. Carter said these talks had ranged over a number of questions, with particular emphasis on the concern of Latin American countries about stable prices and better tariff treatment for their raw materials exports such as copper, sugar and coffee.

However, Carter did not mention what, if anything, was said about the sentitive question of human rights in Latin America. With the exception of Columbia, each of the countries whose representatives met with Carter yesterday is ruled by a military dictatorship - a situation that has led to charges that receiving the leaders at the White House gives a stamp of legitimacy and U.S. approval to their regimes.

In anticipation of this critism, some State Department officials had argued unsuccessfully that the President should avoid private meetings with his visitors. Their concern was underscored yesterday when public criticism of the President's action began snowballing.

Most of the criticism was directed at Chile's Pinochet, who has been the principal target of hemispheric liberals since he came to power in a 1973 coup that overthrew Marxist President Salvador Allende. The Pinochet government has since been charged with killing, torturing and imprisoning domestic political dissidents.

In an echo of this criticism, the widow of Orlando Letelier, a cabinet officer in the Allende government who was murdered in Washington last year, and Rep. Ronald V. Dellums (D-Calif.) called a press conference to charge that Carter was wrong in giving a personal White House welcome to Pinochet and other "dictators."

Isabel Letelier charged Pinochet with "personally ordering the assassination" of her husband. However, the Justice Department has said that its investigation of the still-unsolved murder has uncovered no evidence implicating the Pinochet government.

A number of Chilean exiles and their U S supports criticized Carter for receiving Pinochet and other military rulers. White House press secretary Jody Powell responded that "we've made it quite clean we do not feel that differences with other nations or, in domestic politics, differences with individuals, should preclude conversations and meetings and frank explorations of areas of differences as well as areas of agreement."

"We think that as a general rule, it's better talking to people than not talking to them."

Disclosure of the treaty texts contained no surprises, since their principal provisions already had been revealed by the two governments. In 62 pages of dry, legalistic language, the treaties hopscotch over a broad range of issues, covering subjects as serious as to use of the canal in time of war and as mundane as the future operation of the Canal Zone's bowling alleys and dry-cleaning plants.

Their main thrust, though, is to give Panama full sovereignity over the canal - but only at the end of a slow, carefully calibrated process that will not see the United States surrender complete control over the 51-mile waterway until "noon, Panama time, Dec. 31, 1999."

Until then, the United States will continue to operate the canal. It also will maintain, for the time being, the 14 U S military bases in the zone, phasing them out at Washington's discretion between now and the year 2000, when Panama assumes responsibility for the canal's defense.

As a further reassurance to those who fear the transfer will jeopardize the U S strategic stake in the canal, The second treaty pledges the United States and Panama to jointly guarantee the neutrality of the canal and its accessibility to all the world's shipping after the year 2000.

The Carter administration contends that this provision gives the United States a permanent right to intervene, diplomatically or militarily, if the canal's neutral status is threatened by a future Panamanian goverment or some other country.

To reinforce this point, the United States and Panama will ask all OAS members to approve and sign a resolution pledging to respect the treaty provisions for guaranteeing the canal's neutrality. This resolution also will be open to signing by "all states of the world."

As soon as the treaties go into effect, the United States will increase the annual rent it pays Panama from $2.3 million to $10 million. Panama also will begin receiving an additional $10 million a year from canal toll revenues, provided that receipts are sufficient to cover this payment and meet other operating expenses.

A systematic transfer of territory and property within the Canal Zone also will get under way, with more than half of the 648-square-mile area passing into Panama's hands almost immediately. The entire zone symbolically will come under the flag of Panama, although the U S flag "may be displayed, together with the flag of the Republic of Panama," at the canal headquarters and "at other places and on some occasions" approved by the Panamanian government.

To operate the waterway, the old Panama Canal Co. will be replaced by a Panama Canal Commission, supervised by a nine-member board of five Americans and four Panamanians. The Panamanian members will be proposed by their government, but appointed by Washington.

Until 1990, the canal administrator will be an American, with a Panamanian as chief deputy. The positions will be reversed for the final decade of the traties. In addition, the United States will increase and train the Panamanians employed by the canal, currently about 75 per cent of the 13,000 member work force, for their eventual assumption of control.

U.S. citizens employed by the canal are permitted to keep their positions as long as they choose or their jobs last. For those who leave or have their jobs eliminated, the treaties provide such options as early retirement, transfer to Civil Service posts in the United States or assistance in finding new jobe.

The treaties also recognize the continued right of canal employees to bargain collectively and to belong to unions of international affiliation.