An American-educated architect whose treason trial is to start Tuesday in Somalia has alleged in a statement smuggled from prison to western human rights organizations that he was tortured into signing a false confession by Somali security police.

Suleiman Nuh Ali, a 1971 graduate of Howard University's school of architecture who has been jailed since September 1982, said police ordered him to sign papers confessing that subversive materials had been found in a police search of his home in the Somali capital of Mogadishu.

"I told them I was not a member to any organization (above nor underground) and that the letters and photocopies are theirs, and that they go to hell," Ali wrote. "After {a} few nights, I was taken from my cell, handcuffed, blindfolded, driven in a land cruiser and taken to a beach. There I was tortured, very badly. Many nights were repeated until I was forced to sign anything they wanted."

Amnesty International, the human rights organization, said last week that its London office had authenticated Ali's 28-page statement, written in English. Amnesty and friends of Ali's in the United States declined to say publicly how the manuscript reached the West.

Somalia's ambassador to the United States, Abdullahi Ahmed Addou, denied the torture allegation. "Whoever wrote that statement, I want to tell you that it's not our character and our policy to torture people. Of course, anyone can write and say 'I've been tortured.' But what proof the person has, this is an allegation that to my knowledge is absolutely out of Somali character."

A report last month by the National Academy of Sciences' Committee on Human Rights, alleging torture and harsh prison conditions for Ali and other political prisoners held there, also drew a heated denial from the Somali Embassy here.

The State Department estimates that there are between 300 and 500 political prisoners in Somalia, which does not tolerate internal criticism of its government.

The scheduled trial has alarmed rights group representatives, who say proceedings in the National Security Court in Mogadishu usually are closed and that executions sometimes are carried out "very quickly" after sentencing.

The New York-based Lawyers Committee for Human Rights said last week that its request to monitor the trial has been ignored by the government of President Mohammed Siad Barre.

Ali is among several Somali intellectuals arrested in the early 1980s and accused of plotting to overthrow Barre's authoritarian government. Twenty are scheduled for trial Tuesday. Besides Ali, they include physician Mohammed Aden Sheikh and economist Mohammed Yusuf Weirah, both former Cabinet ministers; Abdi Ismail Yunis, a mathematician and graduate of the State University of New York at New Paltz, and Farah Hussein Ahmed, an engineer.

Several of the prisoners have been held incommunicado and without being charged for years, according to the rights groups.

Amnesty said it believed that since his arrest, Ali had been held in solitary confinement in the basement of the National Security Service building in Mogadishu and in an underground maximum security prison at Lanta Bur, about 15 miles outside the capital.

"I could not sustain the torture till death, so I decided to sign what they had written," Ali said in describing his imprisonment. "The firing squad is better than the torture."

Ali suggested that fellow inmates could testify at his trial to his having been tortured.

"They saw me taken away handcuffed, blindfolded, and saw me returned unconscious, carried by four soldiers, dripping blood, vomiting and near death. The distance between the door of interrogation room and the doors of some of the cells across the dividing corridor is less than 1.2 meters" -- about four feet.

"Not a mattress, nor blanket, not even a plate for our use," he added. "We are denied sunshine, fresh air, exercise. For these almost six years we are in isolation concrete cells contained within a main hall."

The National Academy of Sciences said Ali was working as a consultant to the U.S. Agency for International Development at the time of his arrest.

U.S. security and economic aid funds for Somalia totaled $66.5 million in fiscal 1987.