When Julio de Pena Valdez, an imprisoned labor organizer in the Dominican Republic, got his first 200 letters of support from members of Amnesty International (AI), the guards gave him back his clothes. When the next 200 or so letters arrived, the prison director came to see him.
Finally the letters totaled some 3,000, and he was released, he later wrote. That was several years ago. Today he is still free, according to Ann Blyberg, chairman of the board of the United States section of AI (AIUSA).
It is this kind of concern that has helped boost support for AI in the US to an all-time high, according to AI members at their recent annual meeting here, the first ever held in the South.
As a result of such interest, the AIUSA is mapping a more intense strategy than ever on behalf of human rights.
On its agenda:
* A stronger push for Senate ratification of the antigenocide treaty signed by the US in 1948.
* An enlarged campaign against the ''systematic'' use of torture in some 60 nations.
* A new public information effort to rally opposition to political killings.
* A renewed publicity campaign against the death penalty in the US.
Of some 5,000 political prisoners ''adopted'' by AI letter-writing groups worldwide each year, some 1,000 are eventually freed, says Jack Healey, executive director of the US section of AI.
Though AI is careful not to claim direct credit in such cases, its growing membership likes to be involved in such direct efforts as letter writing to help free political prisoners.
''The fact that people are arbitrarily arrested, often tortured, and executed'' was why Baltimore switchboard operator Patricia Ruck joined AIUSA.
''We all hear about major issues in the news, but feel powerless to act,'' says another active member, New York writer and opera singer John Cimino. AI worldwide has helped free some 14,000 prisoners, ''one at a time,'' he says.
Based in London, Amnesty International began in 1961 and now has some 400,000 members in some 41 nations. The US section has seen its membership grow in the last two years from about 100,000 to 200,000. Donations have increased to nearly
The number of letter-writing groups in AIUSA have increased from about 100 to 240, involving more than 10,000 members, says AIUSA press officer Larry Cox.
The rapid growth in support may be due in part to an impression among some people that ''human rights is being given a lower priority'' by the Reagan administration, he says. AIUSA officials say there was a big jump in membership shortly after the 1980 presidential elections, and that support has continued to grow.
Members cover the political spectrum and most occupations, says Mr. Cox.
The urgency of the world's human rights problems was made clear by survivors of political violence who told their stories to AIUSA members at their annual meeting.
Faran Ferdowsi, an Iranian in exile, described years of systematic repression in Iran of members of the Baha'i faith, of which he is one. Imprisoned with his father, who was later executed, he witnessed many beatings of members of his faith who would not renounce their beliefs. He later fled the country after being released.
Some 140 prominent Baha'i members have been killed since 1979, he alleged. The persecution continues, he said. ''My only hope is that the world will not watch indifferently.''