A firing squad executed 15 Ugandans convicted of plotting against President Idi Amin today as they stood tied against sand-filled water drums near Kampala's clock tower.

The executions were accompanied by Amin's warning broadcast over Radio Uganda that anyone engaging in subversive activities "will actually be committing suicide."

The broadcast made no mention of the whereabouts or health of the 51-year-old Ugandan dictator, who underwent surgery at a Kampala hospital Wednesday. But there was mounting speculation in neighboring capitals and elsewhere that Amin had inspired earlier reports about his lapsing into a post-operative coma to divert internatioanal attention away from today's executions.

A large crowd gathered this afternoon to watch the spectacle. The 15 men were lined up in the same spot where Uganda's first public executions were carried out in 1973.

Twelve of the executed men were convicted last month of having planned a coup to topple Amin. The other three were sentenced in July on treason and murder charges.

Amin did not heed last-minute appeals for mercy by Liberian President William Tolbert and Gabon's President Omar Bongo. Several other black African and Moslem leaders joined the appeal and prostested the executions during the Moslem religious period of Ramadhan.

Among the executed men were Abdalla Anyuru, former chairman of Uganda's public service commission: Y. Okoth, former chief inspector of schools: Elias Okidimenya, former general manager of a Uganda bottling company: and several former officers.

Amin, on e of the most controversial leaders on the world scene, has ruled his country with savage ruthlessness since he assumed power in a 1971 military coup. His opponents maintain that nearly 300,000 Ugandans have been killed in various ways during his 6 1/2-year-old reign of terror.

Meanwhile, a Ugandan government official contacted by telephone said that Amin had left Kampala's Mulengo Hospital and is resting on an undisclosed island in Lake Victoria, according to a UPI dispatch from Nairabi, Kenya.

Yesterday, one of Amin's top aides, British-born Robert Astles, told The Associated Press in Nairobi that Amin was "in a coma" after he was operated on by a Soviet medical team on Wednesday.

"It is serious," Astles was quoted as saying. "We believe he'll pull out of it. We don't know exactly (what's wrong). But he is in a coma."

Western residents of Kampala were quoted by UPI today as saying that a film of the operation was shown on Ugandan television last night.

According to these Westerners, the film showed the sugery consisting of a small incision on the back of Amin's neck, which lasted only a few minutes. They said that Amin was conscious throughout and that the film showed him getting up and walking away from the operating table once the surgery was over.

Another indication that the Ugandan president for life had perpetrated a hoax came from the French ambassador in Kampala, Pierre Renard, who was quoted by Rado Tele Louxembourg in Paris today as saying the reports of a coma "are pure fantasy."

The source of he report, Astles, a white Briton who holds Ugandan citizenship and the rank of a major, is one of Amin's confidants.

Astles is married to a black Ugandan woman who is the country's minister of culture. Last year Astles was imprisoned briefly by the Ugandan president for rumor-mongering.