The Anglican archbishop of Uganda and two government ministers have been arrested in connection with an alleged plot to overthrow President Idi Amin, Uganda Radio announced today.

Archbishop Janani Luwum, Internal Affairs Minister Oboth Ofumbi and Water Resources Minister Lt. Col. Erinayo Oryema were arrested only hours after a rally that Luwum attended where thousands of troops shouted, "Kill them, kill them!" at the accused conspirators.

The radio said all three had been arrested "pending investigation into their role in the plan to cause chaos in the country."

"They will be dealt with according to the law," the radio said. "The arrests should not cause alarm because their cases will be properly investigated."

(Many Ugandans already have been killed and hundreds arrested in the latest purge of Amin's bloody, six-year rule in Uganda, David Martin of the London Obselver reported earlier from Lusaka, Zambia. Touched off by discovery of the alleged plot to kill Amin on the anniversary of his seizure of power, the main purge reportedly is against the Acholi and Langi tribes, with Protestant and Catholic leaders and other opponents of the government also on the list. Martin quoted a Ugandan source as saying that, "Everything points to the great-colossal scale since the regime asset single campaign of massacre on a sumed power.")

Amin has survived an estimated 10 coup attempts since Jan. 25, 1971, when he seized power in a bloodless coup.

The International Commission of Jurists has estimated that the number of Ugandans killed in reprisals for alleged plots against Amin and in purges of his opponents may have ranged as high as a quarter of a million.

There have been reports that Amin has on occasion announced nonexistent plots against him as an excuse to get rid of opponents.

There is little doubt, however, that there have been real attempts to overthrow Amin. Ugandan sources have reported that the president is facing insurrection from nearly half of his military forces. A fourth of his troops are under detention as are virtually all of his top commanding officers.

Amin trucked about 3,000 loyal troops from every army unit in the country to a bizarre ceremony in the capital where he announced details of the plot and implicated the United States, Britain and Israel in the alleged coup attempt.

Three men who said they plotted to overthrow Amin read confessions in the open-air ceremony outside Kampala's Nile Gardens Hotel. They were Aballa Anywui, former head of Uganda's public service commission, John Olobo, a Ministry of Labor official, and Lt. Ben Ogwang of the Ugandan army.

Archbishop Luwum and other Anglican bishops attended the confession ceremony in their purple robes. The archbishop shook his head sadly as the soldier loudly denounced him.

Amin summoned the archbishop earlier in the week and accused him, former Ugandan President Milton Obote, President Julius Nyerere of Tanzania and a second bishop, Yona Okot, of being involved in the plot. But on that occasion he released the archbishop with an admonishment to "preach the word of God and not bloodshed."

Addressing his troops today, Amin said those involved in the coup attempt would be judged at a "proper military trial by a military court. The case will be judged properly before any sentences will be carried out."

Amin displayed 60 Chinese-made rifles, ammunition and hand grenades and 22 suitcases and steel boxes that he said the material was found in.

In a separate statement, an official said the government had proof that overthrown Amin - one in Tanzania and one in Kenya.

He said the Kenyan group was supported by the United States and received funds from the CIA. He said this group had been supplied with arms and training by Britain and Israel.

The All Africa Conference of Churches today sent a telegram of Amin expressing "distress over current reports of a new wave of massacres of Uganda," Reuter reported from Nairobi, Kenya. It asked him to allow a conference delegation to visit Uganda to investigate allegations that Archbishop Lunum had been involved in a plot to overthrow the Amin government. The church group also announced that it had received a copy of a letter addressed to Amin by Archbishop Lunum and 13 other bishops in Uganda that spoke of many killings.