THAT Tibet would erupt was inevitable. Love between the Tibetans and their Chinese masters is as thin as the Himalayan air on the so-called ''roof of the world''. The political air was becoming more rarefied by the day due to the arrest of a number of dissidents, Tibetan suspicion of the Chinese traders who had been flocking to Lhasa, rampant inflation and, according to one overseas-based Tibetan group, a recent purge by the region's new Communist Party boss. Ironically, the presence of a European Community delegation, in Tibet to investigate the human rights situation and conditions, only helped charge the atmosphere. According to the Washington-based Campaign for Tibet in a report last February, Tibet's new Communist Party secretary, Mr Chen Kuiyuan, alienated many Tibetans by purging the government of Tibetan officials who sympathised with the Dalai Lama, the exiledspiritual leader of Tibet and one of China's most bitter foes. In their place, he installed hundreds of Chinese cadres. In a fashion reminiscent of the Cultural Revolution, Mr Chen labelled his political victims ''reactionaries'', reported the Campaign for Tibet, citing what it called a ''highly classified speech''. Chief targets were the offspring of Tibet's former ruling class who had been rehabilitated after the Cultural Revolution and appointed to key administrative jobs. Mr Chen criticised some officials for hanging photographs of the Dalai Lama in their homes. The Washington-based advocacy and monitoring organisation said Mr Chen's speech was ''an indication that the Chinese policy of creating a loyal class of Tibetan administrators through education and appointments had largely failed''. Tension mounted earlier this month with the arrest of at least several dissidents, although three international human rights groups said perhaps more than 100 were detained. In meeting European Community ambassadors, Tibetan officials confirmed the arrest of four specific individuals. They also admitted there were some other arrests, but gave no figure. Among those arrested were Gendun Rinchen, a former tour guide, and Lobsang Yonten, a former monk, who were accused of stealing state secrets and who will apparently be tried. If convicted they could be executed. It appears their arrest was connected to the ambassadors' visit. The London-based Tibet Information Network said it had obtained a letter written by Rinchen which showed he planned to present the delegation with a letter on human rights violations in Tibet. Among other things, the letter said police tortured detainees. Security during the ambassadors' visit was tight. The diplomats were never allowed out of the sight of officials. ''[We] were never alone, and in any talks, there were more officials on the host side than on the other side,'' said one diplomat. A European woman who had been visiting Tibet said everything suddenly changed about two weeks ago. Early in May, the police presence was visible but fairly relaxed. Many policemen simply sat around, some playing cards. About the time of the ambassadors' arrival and in the days before the demonstrations, convoys of military trucks began moving in and out of Lhasa, and new military camps were set up on the outskirts of the city. Police started carrying walkie-talkies. ''There was a feeling that the town was very different,'' the woman said. Tibetans told foreigners of an increased police presence in the week preceding the demonstrations. They said reserve army units and police reinforcements had been drafted in from surrounding areas. Tibetan and Han Chinese plain clothes agents were said to be closely monitoring every aspect of life in the capital. It was not clear whether the tightened security was connected with the delegation, or was a preparation for last Sunday's 42nd anniversary of the 17-point agreement under which the Dalai Lama surrendered Tibet to Beijing. A Swiss national who has been in Lhasa for two months said things began to change three weeks ago. In part, the tension was linked to the arrests, he believed. But it was also evident Tibetans were furious about the Chinese, apparently mainly from neighbouring Sichuan province, pouring in to set up shops. Perhaps these Chinese were simply seeking new opportunities for business, but the Tibetans saw their presence as part of a Chinese plot to tighten Beijing's control of Tibet. The migration of Chinese to Tibet had been going on for more than one year, travellers said, but had increased rapidly in the past few months. So had inflation. After the government suddenly raised prices for petrol, traders increased prices for yak meat, butter and tsampa, the three staple foods of Tibet, according to a Western source quoting Tibetans. Tibetans believed the Chinese settlers would receive special subsidies, enabling them to bear the inflation. The ambassadors have filed a report to their respective governments. The contents of the report have not been made public. According to one source, the diplomats had different opinions about the political atmosphere in Tibet during their trip. Some foundit tense, others relatively relaxed. Clearly those who perceived tension had the best skills of observation.