The United States, in a break with its past practice, has cleared the way for an invitation to the Palestine Liberation Organization to attend President Carter's speech here Thursday night and a reception for the President afterward.

Although the PLO has observer status at the United Nations, the United States has refused to grant it official recognition. In the past, the United States has systematically excluded the PLO from invitation lists to American diplomatic functions here and elsewhere.

Arab diplomats immediately interpreted the move as an American gesture to the PLO. Israeli officials here issued a statement voicing unhappiness.

Spokesmen at the White House, State Department or the U.S. mission here would not comment on whether the move was intended as a gesture to the PLO, but U.S. officials said privately that the decision was made at "the highest levels" in Washington.

The invitation was officially extended to the PLO and 10 other permanent observer delegations at U.N. headquarters by Secretary General Kurt Waldheim. Waldheim acted only after the United States initiated a call notifying him that it would have no objections to such a move, according to U.N. and American officials here.

The White House, presumably to minimize the publicity impact of President Carter shaking hands with the PLO representative, asked the United Nations to cancel planned coverage of the reception by television and photographers, according to U.N. officials.

The acting chief of the PLO observer mission, Hassan Abdel Rahman, told by reporters of the invitation, said he would have to consult with his superiors before deciding whether to attend the Carter speech at the General Assembly chamber and the reception given by Waldheim in Carter's honor in the U.N. Delegates' Lounge.

Until yesterday, only the 146 U.N. member-states had been invited to the speech and the reception. U.S. officials here had admitted that the observers were excluded because of the awkward situation presented by the PLO. The United States had voted against the granting of U.N. observer status to the PLO in 1974.

The other 10 observer delegations all recognized by the United States, represent North Korea, South Korea, the Vatican, Monaco, Vietnam, Switzerland, the European Economic Community, the Arab League, the Organization of African Unity and SWAPO, the chief liberation movement in Namibia (Southwest Africa).

Representatives of South Korea, Switzerland and the Vatican had put pressure on Washington and the United Nations to get invitations to the Carter appearance, according to U.N. and American officials.

U.S. Ambassador Andrew Young reportedly was anxious to get an invitation for SWAPO, as part of the U.S. effort to promote better relations with the liberation movements in southern Africa.

The United States could have invited only the observer organizations it recognizes but Washington apparently chose to signal approval for a blanket U.N. invitation instead, allowing the PLO to attend.

The deputy U.S. representative to the United Nations, W. Tapley Bennett, Jr., who informed the United Nations of Washington's decision, said the United States had simply told Waldeheim that it was his decision because "it's his building."

U.S. officials said privately that the decision was made in the "full awareness" that it would involve an invitation to the PLO.

An Israeli spokesman here said his government was unhappy about the invitation, but would have no comment other than the reminder that "our position on the PLO, which is the only organization in the world committed to the destruction of a nation, namely Israel, has been made clear."

Carter's half-hour speech, scheduled to be broadcast live at 7:30 p.m. Thursday, is expected to cover American relations with the United Nations, human rights, and issues such as the middle East, southern Africa, and the economic relationship between the industrialized nations and the Third world.