"Vietnam is just like a closed and isolated box where people are like little ants," said the former Saigon student leader who had opposed the regime of president Nguyen Van Thieu only to be jailed by the Communists after the fall. "The Communists can pick up anyone they wish, to kill them at any moment they wish; and nobody will know about it."

Doan Van Toai told about 70 largely sympathetic listeners at the National Press Club yesterday that he was chained hand and foot and forced to live in a 12-by-30 foot cell with 40 other prisoners. Every day, said Yoai, someone died from torture, starvation, suffocation or suicide in the Saigon jail. A Vietnamese gulag, he called it, echoing Russian novelist Alexander Solzhenitsyn's accounts of Stalin's prison camps.

Since he was released and allowd to immigrate to France a year ago after 28 months of such confinement, Toai has been waging, in effect, a one-man war of information-his detractors call it propaganda-about the human rights abuses of the Hanoi government.

"Most other prisoners speak just for themselves," said the speaker who introduced him. "Doan Van Toai speaks for his fellow prisoners as well."

He attracted considerable attention in the French and British press earlier this year when he presented a copy of a document he said had been signed by 49 prisoners in Vietnam, including several former members of the National Liberation Front.

"Look at the hell of Vietnam," says this "Testament of Patriotic Prisoners" as it recounts "unthinkable" conditions in packed prisons across the country. Even the numerous jails of the Thieu regime were inadequate for the Communists, according to the document, forcing them to convert schools, hospitals, and orphanages into markeshift prisions.

The testament concludes dramatically with a plea for the International Red Cross to send each prisoner a eyanide pill. "Help us to die at once. We shall be eternally grateful to you."

One of the signatories, Vietnamese writer Nguyen Huu Hieu, escaped from his homeland in May after 10 months of confinement and was with Toai yesterday to support and reinforce his charges against the current regime.

Critics of Toai, principally the Associations of Vietnamese Patriots in Canada and France who are sympathetic to the Hanoi government, have pointed out in their publications that Toai has never produced an original copy of the testament that was supposed to have been circulated secretly through Vietnamese prisons between August 1975 and October 1977.

Toai said yesterday that he had the original when he left Vietnam, smuggling it and several other papers out hidden in his rectum, but that he lost it during the trip and had to rewrite it from memory.

Toai maintains, on the basis of conversations with prisoners who were brought to his Saigon jail from incarceration elsewhere in the country, that there are some 800,000 political prisoners in Vietnam.

Spokesmen for both Amnesty International and the U.S. State Department said that they believe, on admittedly sketchy evidence, that the figure is probably much lower, with one State Department official estimating from 150,000 to 200,000. The Vietnamese admit to between 40,000 and 50,000, the official said.