Cuban President Fidel Castro's angry attack on the Reagan administration yesterday was "an insult to the United States" that "we deeply resent" and "will not forget," Sen. Robert T. Stafford (D-Vt.) told delegates to a parliamentary conference here today.

Castro opened the 68th meeting of the Inter-Parliamentary Union with a blistering address calling the Reagan government "fascist" and "covered in blood," and accusing it of biological warfare against Cuba and genocide in El Salvador.

The Cuban president went on to criticize U.S. policy all over the world and, in the process, insulted most of the Western delegates here and their allies in developing nations.

In what was by comparison a tempered response, Stafford, who leads the small American delegation here, said in a speech to the assembly that Castro's charges were "without factual basis," adding, "yesterday's outrageous 'lecture' causes me to wonder whether this organization is not being transformed into a propaganda forum for individuals who do not share the same regard for truth and decency as those truly experienced in genuine parliamentary traditions."

Stafford said, "The purpose of the IPU is to foster mutual understanding and to seek, by means of dialogue, to attain the common objective of world peace."

The United States found itself backed up by Britain, whose delegation said it was "greatly offended" by the remarks Castro made about Northern Ireland. Canada, West Germany and Egypt also condemned Castro's speech.

At a reception last night, Castro was kidded by a delegate about the women who flocked around him. "Women all admire you," said the delegate. Castro smiled. "Not Thatcher," he said, referring to British Conservative Prime Minister Margaret Thatcher, whom he had called a "tyrant" in the morning.

The Inter-Parliamentary Union meets twice a year in different countries around the world. The current conference is expected to continue into next week.