Leading Panamanian civilian politicians in a joint statement yesterday repudiated the Panama Canal treaties, asserting that in the absence of a new plebiscite "the military government is making this pact on its own."

The opposition parties, which have been illegal since the government of Gen. Omar Torrijos took over a decade ago, issued a "manifesto to the nation" saying that the U.S. Senate's reservations and amendments had "substantially changed" the treaties approved in last October's plebiscite.

"If the government unilaterally approves the amended texts, it violates the constitution," the statement said.

The disavowal of the treaties raised questions about Panama's attitude toward the pact if and when the military government is replaced by civilians.

Spokesmen for four political groupings said last night that should the parties come to power they will immediately renounce the treaties. The parties include the Panamaista Party, the Liberal Party, the Christian Democratic Party, and the "Movement of Independent Lawyers."

Opposition leaders acknowledged at a news conference that there was "little hope" that a new national plebiscite would be called. "But we are making it clear that if there is no plebiscite, the political parties are disavowing the treaties and the military government is making this pact on its own," a spokesman, Carlos Ivan Zuniga, said.

Torrijos has told reporters that no plebiscite was needed in Panama because the treaties "are a triumph and we can live with them." He declared yesterday a national holiday.

Although the traditional political parties have no legal status since 1968, they have acted as pressure groups since Torrijos permitted more political freedom to debate the treaty eight months ago. They are regarded as responsible for mobilizing the anti-treaty vote in the plebiscite.

Two very important groups, however, the Communist Party and the conservative business community, are quietly supporting the treaties now that the Senate has stated the United States does not intend to interfere in Panama's internal affairs.

The pro-government Communist Party largely controls the labor unions while business support is essential to help lift Panama out of its present economic deadlock. Over the last three years the economy has been at a standstill and unofficially, 20 percent of the work force is unemployed.

Neither the businessmen nor the Communists have made public statements so far, but in an interview, Pedro Rognoni, president of the National Council of Private Enterprises, said that although the U.S. non-interference statement was only a "half-hearted solution, it is satisfactory to us. When have often disapproved of Torrijos' actions but we must recognize that he has obtained a new treaty for Panama."

Rognoni indicated, however, that the business community's support had some heavy strings attached to it. The private sector, he said, would now demand "participation in the government at Cabinet level and a strong voice in planning the economy."

"We now want to come out of the closet without being scared and we no longer want to be dismissed as oligarchs," Rognoni said.

Although yesterday was a national holiday, there was no further sign of celebration after Tuesday night's officially organized festivities, which were helped along with generous doses of government supplied rum, whiskey and cane brandy.

The nationalist and leftist student groups who staged anti-treaty protests in recent days also took a rest yesterday, but student leaders said they would demonstrate again today against the "bastard treaties" and the "treacherous amendment."

There is none of the euphoria that might have been expected now that the hated 1903 canal treaty has been replaced by a pact that will make the canal Panamanian in the year 2000.

Panamanians commented with some surprise on Torrijos' strongly anti-American speech at a news conference following Tuesday's Senate approval of the canal pact.

The general not only bitterly criticized the American treaty opponents but also said he would have used violence to close the canal if the treaty had not been approved. Longtime observers here say they are doubtful that Torrijos was actually planning "to destroy the canal" as he said, but they recognized that the government would have felt obliged to express its anger strongly.

"It's not unnatural that Torrijos is blowing off steam against the Americans," one prominent Western diplomat said, "he has spent months bowing and kissing the hands of visiting American senators, swallowing abuse. And all along he's had to keep his mouth shut for fear of losing points."