To the soldiers guarding the Vietnamese-controlled checkpoint east of here, Thach Sang was just another elderly Cambodian searching the border area for long-lost relatives.

In fact, like hundreds of other Cambodians every week, he was escaping the Vietnamese-installed government in Phnom Penh and making his way to one of the western border enclaves outside its control.

But Thach Sang is no ordinary refugee. At age 70, having spent four years as a political prisoner in both Vietnamese and Cambodian jails, he is joining Cambodia's main noncommunist resistance to fight Hanoi's takeover of his country.

"I am determined to die in a liberated area," he said. "So the only thing left to do is to go on fighting the Vietnamese."

Thach Sang's account of how he came to this point not only illustrates one man's personal commitment to a political cause, but also provides a rare glimpse into the rigors of prison under the Vietnamese and their proteges in Phnom Penh.

Thach Sang, a Khom, or ethnic Cambodian born in South Vietnam, fled to his native land to escape the communist Khmer Rouge, who took power in Cambodia in April 1975. A former chairman of a Khmer Khom association, Thach Sang said he was arrested by the Vietnamese in March 1977 on charges of opposing the Hanoi government.

He said he was released in December 1980, underwent a couple of months of medical treatment in Ho Chi Minh City (formerly Saigon) and made his way to Phnom Penh in February of this year. Ten days after he arrived, he said, he was arrested again, this time by the Phnom Penh government under President Heng Samrin.

Accused of being a supporter of Son Sann, the former Cambodian prime minister who heads the main noncommunist resistance group, Thach Sang said he was held for three months and then released for lack of evidence and because he already had served time in Vietnam. In fact, he is a close friend of Son Sann, a fellow Khmer Khom who used to send him letters and money from Paris when Thach Sang was in prison in Vietnam.

"Jail in Saigon was harder than in Phnom Penh," Thach Sang said in an interview at this camp on the Thai-Cambodian border, an enclave controlled by Son Sann's Khmer People's National Liberation Front. "I was tortured in Saigon but not in Phnom Penh."

He said he was beaten during interrogation by the Vietnamese with metal rods and struck repeatedly with rifle butts. In addition, he said, he was subjected to water torture in which guards forcibly poured soapy or salty water down his throat.

Thach Sang said he was held in the Chi Hoa Prison in the Cholon section of Ho Chi Minh City. He said he was sent to do hard labor near Bien Hoa, northeast of the former South Vietnamese capital.

"Over 1,000 Cambodians are detained there and at many other places," Thach Sang said. He said 4,000 prisoners were held at Long Khanh near Bien Hoa.

One of the Cambodian prisoners still held in Ho Chi Minh City, he said, was Son Sann's youngest brother, a former pilot in the pre-1975 Cambodian Air Force under the government of Lon Nol.

Thach Sang said that many other members of the Khmer Khom ethnic group have been arrested in Phnom Penh by the Heng Samrin government, which was installed by Vietnamese invasion troops in January 1979.

The authorities "think that since Son Sann is also a Khmer Khom, they are all his supporters," Thach Sang said. In fact, he said, the 70-year-old resistance leader is popular among Khmer Khoms in both Phnom Penh and southern Vietnam.

After his arrest in the Cambodian capital, Thach Sang said, he was again beaten during interrogation, but not as harshly as in Vietnam. He said he was held in the Central Prison behind Phnom Penh's Onalom Temple along with 4,000 other prisoners.

He was given only rice and salt to eat and lived in a crowded cell with 50 other prisoners, he said. Thach Sang said that most political prisoners held by the Heng Samrin government were accused of backing Son Sann's group, but that he had no idea how many actual supporters were imprisoned.

The Cambodian refugee said the prison warden was a Vietnamese, and that the guards were all Vietnamese soldiers.

Another Phnom Penh prison, he said, was located near Lon Nol's former house and held Vietnamese deserters.

In an indication of the Phnom Penh government's acute shortage of managerial talent, Thach Sang said that when he was arrested he was offered a job as director of Cambodia's national bank.

To reach this camp about 15 miles north of the Thai town of Aranyaprathet, Thach Sang and a relative, Ith Phin, pretending to be smugglers, set out by train to Battambang. Then they headed for the border by motorbike and on foot, reaching here Sept. 15, three days after they left the capital.

Ith Phin, looking older than his 50 years, said his daughter was arrested as an agent of the Khmer People's National Liberation Front 10 months ago and is still being held.