President Carter joined the Panama Canal treaty negotiations yesterday to urge quick completion of the new accord, which faces strong opposition in Congress.

Meeting with American and Panamanian negotiators at the White House, the President said he was pleased with progress so far and with Panama's "very constructive attitude." The United States "will cooperate to the fullest degree," Carter declared, "to rapidly conclude an agreement for a treaty."

Secretary of State Cyrus R. Vance said later that "the most complicated and difficult" remaining obstacle to treaty settlement is the economic terms.

Panama initially asked for about $5 billion in various U.S. payments for continued but lessened American control of the canal until ownership reverts to Panama in the year 2000. Americans sources said yesterday that more realistic amounts are now being discussed but "our figures are still far apart."

Other issues unresolved in devising a substitute for the existing 1903 treaty, negotiators said, include the extent of land and water to operate and defend the canal, and rights of Panamanian workers in the 10-mile-wide Canal Zone.

Panama would gain legal jurisdiction over the canal zone within three years and a share in operating the waterway. After the year 2000, under a proposed parallel treaty, the United States would have a right to intervene if access to the canal is threatened by outside forces.

President Carter is anxious to sign the accord in time to bring it to Congress before the fall recess, to avoid the emotions and politics of an election year. That timetable is in doubt. Vance declined yesterday to set any date of the process.

Administration sources said no U.S. counterproposal on economic differences was presented at yesterday's 90-minute White House meeting, but Panama's economic specialists will return to Washington next week. In addition, a Latin conference on the U.S. Panamanian negotiations has been scheduled next Friday in Bogota. Colombia, with the Panamanaian government chief, Gen. Omar Torrijos, the presidents of Colombia, Venezuela and Mexico, and the prime minister of Jamaica.

Participants in yesterday's White House meeting included Vance, Vice president Mondale, national security affairs adviser Zbigniew Brzezinski, Ambassadors Ellsworth Bunker and Sol Linowitz, and Robert Pastor of the National Security Council staff. Panama's representatives were negotiatiors Aristedes Royo and Romulo Escobar, and Panama's ambassador to Washington, Gabriel Lewis.

Escobar told reporters his country is not considering Carter's recent comments about a possible new sealevel canal through Panama. "We are concerned with our sovereignty." Escobar said: "We would not even consider the problem of a new canal."

At a hearing before the Senate Judiciary Subcommittee on Separation of Power yesterday, Sen. James B. Allen (D-Ala.) charged that in the Panama negotiations the administration seeks to "present the Congress with an accomplished fact and then say, 'Now you have to administer it with whatever powers you have.'"

Criticism of administration strategy by Allen and Sen. Orrin G. Hatch (R-Utah) followed testimony by State Department legal adviser Herbert J. Hansell. Hansell said "we conclude that the President and the Senate, in the exercise of the treaty power, have concurrent power with the Congress to transfer property belonging to the United States."