Three decades after China's Red Army annexed this strategic gateway to central Asia, Tibetan separatists continue to resist communist rule with violence and political subversion.

A small but influential underground that claims a large public following keeps alive the spirit of Tibetan independence despite a generation of communist pressure to control a people with few cultural or political ties to China. The land that once inspired the myth of Shangri-La has become a troubled colony where seven Chinese travelers reportedly were robbed and hanged by Tibetans last spring, where independence activists complain that 2,500 of their confederates languish in communist jails and where dissident youths secretly distribute literature calling for the eviction of Chinese occupiers who make up just 5 percent of the 1.9 million population.

The opposition is believed to pose no real challenge to communist control here, but it is destabilizing enough to draw away large numbers of troops from China's sensitive southwest frontier that abuts Soviet-allied India and lies close to Soviet-occupied Afghanistan.

Tibetan dissent also could have diplomatic costs down the road for Peking, which is anxious to demonstrate to capitalist Taiwan how the communist government can absorb a region with a different political and social system peacefully.

Peking eased its controls on Tibet in 1980, after admitting that past efforts to force socialist modernization and Marxist ideals on the devoutly Buddhist people here brought only suffering, hunger and the systematic destruction of their religion and traditions.

Communist officials now say current reforms that allow greater religious freedom and foster economic development have stabilized Tibet. Any talk of self-determination is no more than a "dirty imperialist trick of aggression."

A recent six-day government-sponsored visit by foreign correspondents to the Tibetan capital of Lhasa showed improvements in energy and grain consumption, medical services, transportation and wages.

But an unofficial view of this Himalayan frontier--pieced together from private meetings with Buddhist lamas, dissidents, other residents and foreign observers--revealed an uneasy society in desperate search of the independence it lost when China occupied it in 1951.

Even the most humane communism is now said to be repugnant to the vast majority of Tibetans, who resent the Chinese for demolishing their monasteries and who prefer to live in the simplicity reminiscent of the Middle Ages, prostrating before holy shrines and spinning decorative prayer wheels.

"Whatever you see in religion is a demonstration of Tibetan nationalism," said a foreign specialist who recently visited Lhasa. "It's their national identification. You never meet anybody who likes the Chinese or the Communists."

Lhasa reflects central government jitters regardless of Peking's public optimism. Thousands of Chinese soldiers patrol the streets and bazaar--part of the estimated 200,000 troops stationed in Tibet. Official posters warn against armed subversion. Foreign journalists were subjected to thorough body searches by airport police looking for dissident leaflets.

Little is known of the resistance movement here. Foreign observers said it has links to Tibetan exiles in India. Estimated numbers range from 100 to several hundred activists.

Although fragmented and unfinanced, the separatists are said to be united in their devotion to the Dalai Lama, Tibet's god-king and former political leader. The Dalai Lama, 49, fled to India in 1959 after an abortive, anti-Chinese uprising. He is still in exile in India.

Pictures of the Dalai Lama appear everywhere--in monasteries, on the walls of primitive mud houses and on the colorful costumes of Tibetans who idolize him as the embodiment of self-rule.

"We are Tibetans and have no freedom, no independence," said a lama who did not wish to be named. "We are not part of China. We do not want the Communist Party as our leader. Communism and Buddhism are in conflict. Our leader is the Dalai Lama."

Dissidents said they maintain contact with the spiritual leader by monitoring Indian radio broadcasts and by obtaining his photographs and tape recordings from religious pilgrims coming here.

Meeting secretly in private homes, they discuss his political and religious tracts, then transmit them to followers. The sayings urge Tibetans to avoid cooperation with the Communists. On the anniversary of the 1959 insurrection, they have covered the old city bazaar with pro-independence leaflets.

Although separatists advocate passive resistance against the well-armed occupiers, they said violence often flares. Chinese reportedly are afraid to walk city streets at night for fear of knife-wielding Tibetans.

The reported hangings of seven Chinese traveling on a small bus in southwestern Tibet remains unsolved, but officials have told foreign visitors that rebels were responsible for the assault.

"The Chinese Communist Party has all the guns, and we don't have a chance if fighting breaks out," said a dissident. "But the two groups clash all the time. Then, the police arrest Tibetans and put them in jail."

When foreign correspondents arrived last week, dissidents were armed only with literature. They roamed the bazaar at night, quickly slipping notes to the visitors and dashing off. Several letters written in Tibetan were addressed to the United Nations from "the people of Tibet."

One letter partly written in English urged Peking to "stop genocide, stop butchery" in Tibet and called for independence.

Three dissidents who spoke in separate meetings said the opposition was forced underground because of a Chinese public security dragnet that has jailed thousands of activists and sent past independence leaders to their execution.

The dissidents said they try to blend into the Tibetan community to avoid detection by police informers. One of them, a middle-aged man, said he hangs a picture of late Communist Party chairman Mao Tse-tung in his house to confuse authorities.

"It's protection," he said. "Everyone has a picture in the house. Otherwise, there could be trouble."

For less cautious activists, there is certain punishment, he said. Among political prisoners named by dissidents are Losang Wangchuk, 65, a writer who cited Mao's works on national liberation to justify Tibetan independence; Tshering Lhamo, 55, who advocated self-determination at a public meeting, and Dawa, who was caught with a speech by the Dalai Lama commemorating the 1959 rebellion.

"Most Tibetans support these people," said a dissident. "The majority will never work with the Chinese because Tibetans won't forget what has happened here."

What has happened in Tibet since the communist conquest was the attempted conversion of an ancient culture that gave this high plateau, known as "the roof of the world," a separate identity and independence from China for most of its history.

Although destruction of some monasteries began as early as 1951, the wholesale trashing of Tibet began in the 1960s. Mao's Red Guards brutally suppressed all religious life and imposed a dictatorship that went to such extremes as forcing Tibetans to abandon their staple of barley in favor of the winter wheat preferred by Chinese.

When the radical era ended in 1976, the costs were irreversible. Only a handful of Tibet's 2,100 temples escaped ruin. Thousands of lamas were persecuted and Buddhist shrines were smashed to dust. Tens of thousands of Tibetans were on the verge of starvation after the winter wheat crops failed in the high altitudes.

"Tibetan people are in danger of being exterminated both as a race and as a culture," concluded the Dalai Lama's sister, Jetsun Pema Gyalpo, after an inspection trip here in 1980.

Prompted by such complaints, Communist Party General Secretary Hu Yaobang went to Tibet for a look in April 1980. He was startled by what he saw and ordered immediate reforms to revive the region.

Since then, 45 monasteries have been reopened and 500 new lamas have taken their vows. Tibetans are now free to display pictures of the Dalai Lama and to practice their beliefs openly.

New, flexible economic policies and government subsidies have helped boost average annual per capita incomes from about $75 in 1979 to $135 last year. The number of Tibetans in local leadership jobs has increased to 70 percent of the total cadre ranks. Official documents are now written in the Tibetan language instead of Chinese.

For many disaffected Tibetans, however, the reforms are poor compensation for years of suffering. As long as they are ruled by Communists, they say, Tibet cannot enjoy a full Buddhist life.

Indeed, the relaxation on religious affairs should not be overstated. The number of new lamas is strictly limited, with novices required to demonstrate their loyalty to the state and party. Although temples have been reopened, their hours for worship are restricted.

"The Chinese claim to have come to liberate Tibetans," said a dissident. "But to liberate someone, you have to give him better conditions in the spiritual sense as well as material. Tibetans are still living under great pressure."

Other sources said the economic advances have been limited to Tibet's cities while rural areas remain badly impoverished, with shortages of food and clothing for the majority of Tibetan citizens.

"Chinese people eat well and dress well because the communist leaders are Chinese," said a lama. "We Tibetans are poor."