The U.N. General Assembly will move swiftly to provide a forum for PLO Chairman Yasser Arafat, who has been denied a U.S. visa to address the body, the assembly's president said Monday.

Arafat, in his first public comments since the U.S. denial Saturday, called the U.S. move "a sheer violation of the international law and the U.N. charter" and said Washington was ignoring international backing for the newly proclaimed state of Palestine."Why are they afraid that I speak to world public opinion and explain the new Palestinian decisions?" Arafat asked reporters in Iraq.

Foreign Minister Dante Caputo of Argentina, president of the General Assembly, said in a written statement Monday that the United States "is under obligation to grant the visa" sought by Arafat.

"As soon as we receive a special request from the Arab group we shall proceed in the best possible manner to ensure that Chairman Arafat is able to address the General Assembly," he said.

The 159-member General Assembly was expected to support overwhelmingly a request from the Arab nations to convene a special assembly in Geneva to hear Arafat. The Arabs were meeting Monday to formulate their request. The special session, unprecedented for the United Nations, would be held later this month or in January.

Also Monday, the U.N. Committee on Relations with the Host Country was to meet to consider the U.S. decision, which U.N. officials call a violation of the 1947 U.S.-U.N. Headquarters Agreement. (Under the agreement, the United States agrees not to impede the transit or work of U.N. diplomats or guests on U.N. business.)

But State Department spokesman Charles Redman said the "decision is firm and final" and that the United States did not violate the U.N. Headquarters Agreement because specific exceptions are made under the accord for "threats to security."

Redman said the United States did not favor a U.N. session in Geneva, citing the large expense that would be involved in such a move, but would not oppose it.

President Reagan lauded the decision, saying, "I think the other way would have sent out the wrong signal - that we are patsies."

Speaking on CBS "This Morning" Monday, a member of the PLO's U.N. delegation called the U.S. charges of terrorism "nonsense."

"It is not serious . . . because he (Secretary of State George Shultz) cannot substantiate his allegations," said the official, Hassan Rahman.

A U.N. statement said that denying Arafat a forum "would be unfortunate at a time when, in the view of the secretary-general, the recent meeting of the Palestine National Council in Algiers provides fresh opportunities for progress toward peace in the Middle East."

The Palestine National Council, the PLO parliament-in-exile, proclaimed an independent Palestinian state during its meeting in Algiers on Nov. 15.

The Palestine homeland proclamation implicitly recognized Israel by endorsing a U.N. resolution that guarantees all Middle East states the right to exist in peace. It promised to restrict guerrilla activity.

The Arab League's U.N. ambassador, Clovis Maksoud, said Sunday he has enough votes to reconvene the General Assembly in special session in Geneva. The non-aligned group has 101 of the 159 voting members in the General Assembly. The PLO has non-voting observer status.

In March, the General Assembly voted 148-2 to condemn the U.S. attempt to evict the PLO Mission by attempting to enforce an anti-terrorism law. Only Israel joined the United States in voting against it.

King Hussein of Jordan and Egyptian President Hosni Mubarak have agreed to spearhead the effort to reconvene the General Assembly.