President Reagan yesterday denounced and vowed to veto a Senate Democratic plan to limit the administration's tanker-escort operations in the Persian Gulf, characterizing it as an invitation to "retreat" that could have "disastrous effects" on U.S. commitments in the region.

Late yesterday the Senate agreed to postpone until Wednesday action on the plan, which was unveiled by Majority Leader Robert C. Byrd (D-W.Va.) and other Senate Democratic leaders Thursday. It would require congressional approval for continuation of the escort operation for more than 90 days after enactment of the legislation.

Reagan, criticizing the plan in a statement, said it "would force a retreat from the Persian Gulf by the United States" and "ultimately . . . could provide a means for Iran to achieve what cannot be achieved by any other means: namely, our complete withdrawal from the Persian Gulf and the Gulf of Oman."

In a separate action, Reagan ordered a ban on sale of U.S.-made scuba gear to Iran, declaring that the underwater equipment would be used in attacks on Persian Gulf oil rigs and shipping.

Democrats had attempted to avoid charges like the ones Reagan made yesterday by targeting only the reflagging and escorting of Kuwaiti tankers rather than invoking the 1973 War Powers Resolution.

In introducing the plan, Byrd said it is intended to "reduce the level of tension and begin to deescalate the conflict in the gulf."

Without mentioning the reflagging and escort operations directly, Reagan said the United States is "making real progress" in efforts to end the Iran-Iraq war through diplomatic means and vowed not to "abandon our strategic interests or our friends" in the gulf region.

"Now is the time to show steadfastness in our commitment, not vacillation and timidity," he added.

Reagan's statement followed an even sharper attack earlier in the day by White House spokesman Marlin Fitzwater, who said the Democratic plan "could have the ultimate effect of achieving the ayatollah's purposes and forcing the United States entirely out of the Persian Gulf and Gulf of Oman."

Fitzwater said the Democratic plan would "pull the rug out from under the United States and our friends in the Persian Gulf" and added, "Iran must be as overjoyed at its prospects for passage as our friends around the world are dismayed."

The Democratic plan was proposed as an amendment to the defense authorization bill for next year, which the Senate is hoping to pass next week. Reagan has targeted the bill for a veto because of arms-control constraints that he opposes.

The White House counterattack came as the Senate, bogged down in proposed amendments to the defense bill, put off a vote on the escort curbs, probably until after other defense issues have been resolved. Senate leaders also faced GOP filibuster threats and were exploring options that included stripping the escort provisions out of the defense bill and taking them up as separate legislation.

But a separate bill could also be filibustered, and Republicans appeared reluctant to guarantee a vote on war-powers constraints now that Democratic leaders are backing them, strengthening their chances of passage.

Faced with the threat of roll-call votes through the night and today, the Senate finally agreed last night to a complicated formula for disposing of dozens of amendments to the defense bill, envisioning debate on the proposals today and Monday and action on Tuesday.

This left two major issues to the end: the Persian Gulf and a proposal to force the United States to resume compliance with the unratified SALT II arms-control treaty, which has also prompted Republican filibuster threats.

At this point, the Senate will be under heavy pressure to resolve both issues as the last stumbling blocks to passage of the defense measure, which has been on the Senate floor for two weeks.

The plan was offered as an alternative to a proposal by Sens. Lowell P. Weicker Jr. (R-Conn.) and Mark O. Hatfield (R-Ore.) to invoke the War Powers Resolution, a move that was rejected last week by a vote of 50 to 41.

But the earlier vote came before the U.S. helicopter attack Monday on an Iranian naval vessel that had been laying mines in the gulf. That attack revived pressure for action to give Congress some control over the operation's scope and duration. However, there is reluctance by Byrd and others to invoke the war-powers law, in part because such action could lead to charges that all U.S. military activities in the gulf are being made subject to termination by Congress.

Although the Democratic plan is aimed exclusively at the escort operation, Republican leaders immediately dubbed it "War Powers II."

Chances for passage of the Democratic proposal were enhanced when Hatfield said he could support it, even though he still favors invoking the War Powers Resolution. Democrats said they think that they have the votes to pass the plan but indicated uncertainty about how to contend with the GOP filibuster threats.