Cuban President Fidel Castro today bitterly assailed "criminal Yankee imperialism" and charged that the Reagan administration had instructed the U.S. Central Intelligence Agency "to resume plans to kill Cuban leaders, especially its government."

Castro spoke as outgoing chairman of the movement of nonaligned nations, which opened its seventh summit here today. In contrast to the Cuban leader's harsh tone, Indian Prime Minister Indira Gandhi, who will serve as chairman for the next three years, opened the meeting of the 101-member organization with a softly worded appeal for global nonviolence.

The passing of the chairmanship coincided with the efforts of centrist forces among the 76 heads of state who have gathered here, along with 3,000 supporting delegates and staff, to begin erasing the pro-Soviet stamp that Castro sought to put on the 1979 summit, held in Havana, when he declared that the Soviet Union was the "natural ally" of the nonaligned.

Meanwhile, as government leaders and their representatives gathered in New Delhi's heavily guarded Vigyan Bhavan conference hall, Egyptian President Hosni Mubarak reportedly met privately for two hours with Jordan's King Hussein to discuss President Reagan's Middle East peace initiative.

Details of the Egyptian meeting with Hussein, who reportedly also is scheduled to meet here with Palestine Liberation Organization Chairman Yasser Arafat, were not disclosed. But summit sources said the discussions included a report on Hussein's efforts to form a Jordanian-Palestinian delegation that would meet with Reagan on a proposed confederation between Jordan and a future Palestinian-governed West Bank.

During a visit to Washington in December, Hussein was said to have told Reagan that he would be able to obtain a commitment from Arafat to begin preliminary talks on a West Bank confederation by March 1. But the continued presence of Israeli troops in Lebanon prompted the Palestine National Council, which met last month in Algiers, to withhold approval of the negotiations, despite U.S. attempts to separate the two issues.

In her speech opening the summit, Gandhi adopted a conciliatory tone toward the developed part of the world and made no explicit mention of either the Soviet Union or the United States. She appealed for a new international economic order to redress inequities, and for a suspension of the testing, production and deployment of nuclear weapons.

She said nonaligned nations would seek friendship with all governments, except those "which are racist or threaten the hard-earned freedom of others."

In sharp contrast, Castro, during a nearly two-hour speech to the opening session, condemned the United States, which he equated with "colonialism, neocolonialism, fascism and racism."

Castro claimed that he had learned from "trustworthy sources" that the Reagan administration had instructed the U.S. Central Intelligence Agency "to resume plans to kill Cuban leaders, especially its government."

In Washington, State Department spokesman John Hughes said Castro's charge was "obviously untrue" and "we absolutely deny it."

Castro also accused the United States of encouraging "execrable adventurism" by Israel in Lebanon, of "aid and abetment" of South African domination of Namibia, of aggression against Libya, of "genocide" in Central America, of military expansionism in the Indian Ocean, and of "irresponsibly" imposing high interest rates on the world market.

However, the tirade by Castro appeared to be viewed by a growing moderate stream in the nonaligned movement as utterances of a ghost of summits past, as most speakers adhered more closely to the tone set in a draft declaration of principles.

The draft, while condemning nuclear proliferation by the superpowers and calling for an overhaul of the international monetary system, makes only two oblique reference to the United States.

The change of tone appeared to reflect growing dissatisfaction with the swing from centrism that occurred during the Havana summit, which prompted Burma to quit the nonaligned movement and Tanzanian President Julius Nyerere to remark to Castro, "I am not sure, Mr. Chairman, that this movement has permanent enemies and permanent friends, let alone natural ones."

A senior Indian Foreign Ministry official who visited 30 nonaligned countries to seek a consensus on the draft resolutions said, "I don't think there will be another Havana conference. There still is a radical representation, but it is out of the mainstream here." He estimated that only 20 of the 101 states represented at the summit fit into the radical mold.

Although a draft of the conference's proposed declarations on the Middle East includes strong demands for Israeli withdrawal from Lebanon and all territories seized in 1967, including East Jerusalem, the summit is not likely to become directly involved in a Middle East peace initiative.

"The nonaligned movement has a basic handicap, and that is that we have been too closely identified with the Palestinian cause and with the Fez declarations to be useful in mediating," an Asian diplomatic source at the summit said.

An Arab source here said that while the movement would not officially involve itself, he expected numerous meetings to be held among the Middle East delegates.

The unexpected arrival here of Lebanon's president, Amin Gemayel, prompted speculation he would meet with Syrian President Hafez Assad and other Arab leaders to discuss the occupation of Lebanon.