CHINA yesterday strongly denied reports of a massive troop build up in Tibet and once again reiterated its firm opposition to the Dalai Lama's meetings with United States President Bill Clinton and Vice-President Al Gore. Foreign Ministry Spokesman Wu Jianmin said reports from the Dalai Lama's Tibetan government in exile that 30,000 troops from three divisions of the People's Liberation Army had been moved into Tibet were ''sheer fabrication''. Mr Wu noted that China and India had signed a border agreement last year and were engaging in talks on troop reductions along the border in Tibet. Despite official denials, reports from several independent sources continue to indicate large scale troop movements into Tibet, concentrated on the Sino-Indian border. Reports claim troops began moving into Tibet from the southwestern provinces of Yunnan and Sichuan at the end of March and could represent the biggest military build up in Tibet since martial law was lifted in Lhasa in 1990.