Senate opponents of the Panama Canal treaties abruptly changed their tactics yesterday and agreed to a final vote on the first of the two treaties in 10 days.

This unexpected break in the three-week-old debate was taken by treaty supporters as a sign the opponents have gives up hope of amending the first treaty and feel that further delay might alienate senators who are considering voting no on final passage.

Sen. Pual Laxalt (R-Nev.), apparently the initiator of the new opposition tactics, said yesterday that the opponents had to avoid being pictured "as stalling on these things," because stalling would alienate some key swing votes.

The decision to go for a vote on the so-called neutrality treaty March 16 represents a gamble for both siders, since neither can now count enough firm commitments to prevail. Protreaty forces feel they have 63 to 64 votes for certain, whole anti-tready senators count 28 to 30 sure votes. It will take 67 to approve the treaties, 34 to block them, if all 100 senators vote.

Laxalt said yesterday he still saw "a 50-50 chance" that 34 senators would vote no. Treaty supporters and White House lobbyists countered with cautious predictions that they would prevail.

The neutrality treaty includes Panama's commitments to keep the canal neutral after 2000, when the waterway would finally pass from American to Panamanian control. Before next week's final vote the treaty is expected to be amended in ways already approved by Panama and the White House to affirm U.S. rights to defend the canal and use it for expeditious passage of warships in emergencies after 2000.

The Senate leadership of both parties is sponsoring these amendments, which have overwhelming support.

The opponents got more votes - 40 - for one of their amendments yesterday than on any previous roll call during the debate. Laxalt and Sen. James B. Allen (D-Ala.), another leader of the opposition, both claimed this was a good sign for the ultimate outcome, but Majority Leader Robert C. Byrd (D-W. Va.) dismissed it as insignificant.

The amendment defeated yesterday would have expressed permitted the United States to block the canal tcships of hostile powers during a future war. A vote to table the idea passed 52 to 40. Virginia's two senators voted in favor of the amendment, while Maryland's senators both voted to table table it.

Though they have agreed to the vote next week on the neutrality treaty, the opponents have not yet said when they would be prepared to vote on the second treaty, which covers the transfer over the next 22 years of the canal to Panama.

Laxalt yesterday referred to this as "the main treaty," Allen said there was "no hope" for an early agreement on an acceptable date to vote the Panama Canal transfer treaty up or down.

However, Sen. Frank Church (D-Idaho), a floor leader for the treaties, said negotiations were continuing to set a date for a final vote on the second treaty, too. Church said Byrd would continue to hold open the possibility of sharply curtailing Easter recess until a date is set.

Laxalt indicated it might be possible to set a date for sometime in early April, after the Senate's return from the recess, now scheduled for March 24 to April 3.

Allen yesterday expressed some annoyance at Majority Leader Byrd's tactics in pressing for a speed-up of the debate. Byrd had warned senators that they would lose most of their Easter recess and might have to stay in session until 8 or 9 p.m., perhaps even on Saturdays, if the debate did not begin to move more quickly.

"This is an old game, you know, "Allen said, "calling off recesses and lengthening the hours of debate."

"Let them meet, I don't care," Allen added. But other opponents did not share that view. "They were under a lot of peer pressure" from other senators to speed things up, one administration official said.

Laxalt invited a group of about 20 treaty opponents to a breakfast strategy session yesterday, and the new tactics apparently emerged from that meeting.

Byrd said public disapproval of the prolonged debate and pressure from other senators forced the opponents' new tack.