In bitterly anti-American and anti-Western rhetoric not heard here since the early 1960s, Cuban President Fidel Castro today attacked the Reagan administration as "fascist," "genocidal," and "covered in blood" for its activities on three continents.

Castro's address at the opening of a conference of the Inter-Parliamentary Union, attended by delegates from 99 countries including the United States, reflected a new low in the increasingly hostile relations between Havana and Washington.

The head of the U.S. interest section here, Wayne Smith, walked out of the convention hall during Castro's speech in protest.

Washington has put increasing pressure on Castro's government, encouraging other Latin American nations to break or reduce diplomatic relations with Cuba and accusing Castro of aiding and arming rebel movements in Central America.

On several occasions, U.S. Secretary of State Alexander M. Haig Jr. has threatened to "go to the source -- Cuba," to end alleged arms shipments to guerrillas in El Salvador and other countries.

Today, Castro took the offensive. He called Washington's allegations "lies," made to justify increased U.S military aid to the Salvadoran government with "a cynicism Goebbels would envy" -- a reference to Nazi propagandist Josef Goebbels.

"It is a lie that Cuba is supplying weapons and ammunition to Salvadoran patriots," Castro declared. "The channels for it do not exist, and the Salvadoran patriots have been fighting for months with their own resources and the weapons they wrest from the enemy. Lies, lies and lies. We defy the U.S. government to present even the slightest evidence to confirm its statements . . . ."

He went on to accuse the Reagan administration of fostering a "maniacal and wild" arms race by refusing to negotiate with the Soviets until the United States has clear superiority, building such weapons as the neutron bomb and MX missile, and deploying new missiles in Europe.

Castro cited a list of what he considered recent U.S. crimes and provocations: "genocidal actions" in El Salvador, aid to Israel despite the bombing of Iraq's nuclear reactor and civilians in Beirut, improving relations with South Africa and condoning its invasion of Angola (where Cuba has a substantial troop presence), and shooting down two Libyan jet fighters over the Gulf of Sidra. Castro also repeated accusations that the Central Intelligence Agency is conducting biological warfare against Cuba. The United States has denied such charges.

The Cuban president did not stop with denouncing the United States.

He went on to attack the Conservative British government of Prime Minister Margaret Thatcher for its policies in Northern Ireland, where he likened Irish Republican Army hunger strikers to Jesus Christ and said "tyrants tremble" before their moral authority.

Castro, whose government is heavily subsidized by the Soviet Union, also criticized Washington's improved relations with the People's Republic of China, which he called a further danger to world peace.

Castro's speech received a standing ovation from many delegates. But the British ambassador and a member of the Chinese delegation joined Smith in walking out in protest before the conclusion of Castro's lengthy speech.

The Reagan administration had attempted to discourage a U.S. congressional delegation from attending the conference, refusing delegates the traditional use of a military aircraft for transportation.

But Sen. Robert T. Stafford (R-Vt.) and Rep. George W. Crockett Jr. (D-Mich.) attended anyway, pointing out that meetings of the Inter-Parliamentary Union are functions they normally attend and that they feared there would be no voice to defend the United States if they were not present.

Stafford said he would have walked out during Castro's speech if he had not been seated with other dignitaries on the stage of the convention hall instead of down on the floor. He called Castro's allegations "unfair, often untruthful, certainly impolite and, in my opinion, undeserved."