President Idi Amin has postponed his meeting with the estimated 240 Americans in Uganda until Wednesday and government officials there said it would take place in the presence of 3,000 persons at Entebbe Airport.

Radio Uganda, which yesterday announced the postponement, also said that a U.S. Naval task force off the East African coast must be seen as a threat to Uganda and that the Ugandan armed forces were told to be on 24-hour alert.

Amin's meeting with Amerians, originally scheduled for today, was delayed because some of the Americans had to travel long distances to reach to newly announced site of the meeting, the main lounge at the airport, the radio said.

The radio said the meeting place was changed from the International Conference Center in Kampala because the airport building was the only one in Uganda that could accommodate 3,000 people.

No hint of who the 3,000 people would be has been given. The last time that number gathered for a meeting in Kampala, most of them were soldiers who heard allegations read out that the Anglican archbishop of Uganda and others were involved in a plot to overthrow Amin.

Archbishop Janani Luwum and two Cabinet ministers were killed in a car crash shortly after the Feb. 16 meeting, according to the Uganda government.

President Carter's subsequent statement that "horrible murders" had taken place in Uganda is believed to have prompted Amin's summons to the Americans in his country, most of whom are missionaries.

Radio Uganda has been regularly repeating assurances that the Americans are in no danger.

In Washington, President Carter said the United States was monitoring the Ugandan situation "closely, trying not to take advantage of his good wishes that he has expressed."

"We've had offers from the U.N. to intercede, but we've handled out affairs through the German embassy and so far there's nothing to cause deep concern," Carter said.

Secretary of State Cyrus R. Vance said that "no alarming conclusion" should be drawn form the postponement of the meeting between Amin and Americans in Uganda.

"We have received assurances from the president, President Amin, and from his government that the Americans will be safe and we are following the situation carefully as it moves along," Vance said on the CBS television program, Face the Nation.

President Carter, winding up a weekend at his Camp David, Md., retreat yesterday, bowed his head and prayed silently when the Episcopalian minister at the nearby Harriet Chapel called for prayers for Amin.

"When Christians begin praying, God begin working, so remember in your prayers Idi Amin," Rev. Charles O. Shaffer, 43, said. The priest also asked the 45 to 50 persons in the chapel to pray for the churches of Uganda.

After church, Carter said he had no comment on postponement of the Ugandan leader's meeting with Americans.

The Ugandan embassy in Washington has scheduled a press conference for 11 a.m. today to discuss "current events pertaining to the situation' in Uganda, an embassy spokesman said.

Meanwhile, a spokesman for Amin denied a Nairobi, Kenya, newspaper report that a Cuban military delegation, headed by Gen. Francisco Carbrera, had arrived in Uganda last week to assess the country's military needs. The Ugandan presidential spokesman, reached by telephone in Kampala, said there was no thruth to the report, Reuter said.

In Washington, Pentagon and State Department spokesman stood by earler comments that the U.S. Navy task force in the Indian Ocean off East African coast has been scheduled to be there long before recent events in Uganda.

"In the events of an invasion of Uganda, the invading force will be destroyed by the Uganda armed forces," Radio Uganda said, in announcing that he country's troops had been placed on alert.

The radio broadcast recalled the airborne rescue mission last year when Israeli commandos freed more than 100 hostages at Entebbe Airport. The operations deeply humiliated Amin.