Prime Minister Zulfiqar Ali Bhutto bitterly accused the United States tonight of financing a "vast, colossal, huge international conspiracy" with his political opponents to force him out of office.

The reason for the alleged conspiracy, Bhutto told a joint session of Pakistan's National Assembly and Senate, is that the United States could not forgive him for failing to support the U.S. role in Vietnam and for supporting the Arab cause against Israel.

The rambling, one-hour-and-45-minute speech, delivered without notes from the floor of the Assembly chamber, was marked by outbursts of fistshaking rage against the United States -- which he termed "an elephant, which does not forget or forgive."

The prime minister's allegations, to which he referred repeatedly, marked the sharpest Pakistani attack on the United States since the Ford administration blocked France's plan to sell Pakistan a nuclear reprocessing plant last year.

Former Secretary of State Henry Kissinger, who met with Bhutto in Lahore, the Punjab provincial capital, last year, said the two governments were trying to reach a compromise. The Ford administration indicated, however, that if Pakistan went through with the purchase, it risked losing U.S. economic aid, which has totaled $4.9 billion since 1952.

Renewed negotiations with the Carter administration are pending. According to a Foreign Ministry source, Secretary of State Cyrus Vance has written to Bhutto proposing that the discussions resume as soon as possible.

A U.S. embassy spokesman refused to comment on two specific claims Bhutto made.

One was that a U.S. diplomat protested to the Foreign Ministry earlier this week over a demonstration in Islamabad by Bhutto's Pakistan People's Party in which marchers shouted "Jimmy Carter Murdabad," which Bhutto said could be translated as either "death to" or "down with."

The other claim was that a U.S. diplomat, in a telephone conversation with one of his colleagues on April 12, allegedly said of Bhutto, "The party's over, the party's over. He's gone," evidently meaning that the opposition would succeed in ousting the premier.

"Well, gentlemen" the premier said to cheers, "the party is not over."

[The State Department strongly rejected Bhutto's charges, calling them "groundless."]

["The U.S. government has neither the desire not the reason to make any effort to support the prime minister's opposition or to interfere in the political process in Pakistan, and it has not done so," a State Department statement said. Under Secretary of State Philip C. Habid protested to the Pakistan embassy, a spokesman said, and the U.S. embassy is to express U.S. concern to the government in Islamabad Friday.]

The prime minister did not indicate how he came to know of this alleged telephone conversation.Two weeks ago, in a news conference, he told a correspondent of Newsweek magazine that he had read his "recent dispatch." The correspondent later said the report had not yet been published.

All correspondents' telegraph and Telex messages are known to be monitored by the government, but foreign reporters have not yet been subject to the censorship imposed on Pakistani news media.

The U.S. embassy spokesman, when asked if embassy personnel assumed that their telephone conversations were tapped, said, "When we are in foreign countries we usually assume that the (communications) systems are not secure."

Bhutto, 49, who has been under severe physical and psychological strain for nearly three months, as a result of vigorous campaigning and trying to cope with the opposition's concerted attacks on him, appeared to lose his train of thought as he spoke. His voice broke frequently.

[Bhutto has recently begun taking sedatives to help him cope with the strain, informed sources in Washington said.]

Some political observers had anticipated that he would use the speech to announce some sort of compromise with his opponents, who have organized violent demonstrations in cities throughout Pakistan since Bhutto won an overwhelming victory in national elections March 7. As many as 300 persons have been killed by police and military forces, and martial law has been imposed on three cities.

Bhutto made no gestures to his opponents, organized under the green banner of the nine-party Pakistan National alliance. All the top leaders of the alliance are being held in a police guest house 12 miles from here. A junior alliance official said tonight that the leaders were listening to Bhutto's speech, carried live by the government radio and television networks.

"I cannot speak as a spokesman for the PNA," said the official, Hasaan Mahmood, "but he has accused us of being Western agents and receiving huge amounts of foreign money. We had expected him to make some sort of offer to settle the troubles. Now it appears he has totally changed his approach."

The prime minister told the joint session that there was "no parallel anywhere in the world" of the vast amount of money the United States allegedly passed to the opposition. Figures commonly heard in Islamabad diplomatic circle gossip range up to $25 million.

"Is it a secret, the amount of funds which have flooded into Pakistan?" Bhutto asked the assembly. "The way people have been bribed -- the postman, the milkman, the man who comes to your house to read your meter -- all have been bribed to attend opposition rallies and support them."

Although he conceded that wealthy Pakistani industrialist families, the so-called 22 families, whose vested interests had been nationalized by the six-year-old Bhutto government, had helped finance the opposition, the premier said these families do not possess the vast sums required.

"I tell you, this is not a basic PNA conspiracy," he said. "This is an international conspiracy." As evidence, he noted that last week's general strike in Karachi and several other industrial cities was called a "wheel-jam" by the labor unions that organized it, suggesting that the wheels of industry were jammed.