The unveiling of the silk tangka on a slope beside the 586-year-old Drepung Monastery came at the start of the Yogurt Festival, a major Tibetan Buddhist celebration. Monks played horns and cymbals and broadcast chanting over loudspeakers as 130 maroon-robed monks lifted a yellow cover, revealing the image of Sakyamuni, or Gautama Siddhartha, the founder of Buddhism.
The picture of the Buddha - 126 feet high, 111 feet wide and sewn from silk by the monks in the monastery - is unveiled each year to great fanfare at the start of the festival.
The annual ceremony first took place shortly after the monastery was founded in 1416, according to Lobsang Wangchuk, one of its senior monks. He said the ceremony was suspended during the 1966-76 Cultural Revolution and resumed in 1984.
During the Cultural Revolution, radical leftists incited by Mao Zedong destroyed thousands of temples and monasteries, killed monks and forced others to give up their religious practices.
China has eased restrictions on religion somewhat in recent decades, but continued controls on Tibetan monasteries prompted rioting in the early 1990s. Chinese authorities still interfere in religious activity, and rights groups say scores of Tibetan Buddhist monks remain imprisoned.
On Thursday morning, older people in traditional Tibetan clothing climbed the mountainside carrying candles or with their hands clasped in prayer. Younger Tibetans wearing baseball caps and sports jerseys snapped each other's pictures against the Buddha backdrop.
Some climbed the hillside to circle around the painting, throwing silk prayer shawls and tiny slips of colored paper bearing prayers onto the image.
Lobsang said 100,000 people were expected to visit the image during the next several days.
In the painting, the giant Buddha, wearing a red-and-gold robe, sits surrounded by images of smaller Buddhas, some wearing the maroon robes and yellow pointed caps of Tibetan monks.
Later in the morning, Chinese authorities held a ceremony in central Lhasa, below the fabled Potala Palace, former home of Tibet's rulers, to open the festivals. Under heavy security, a military band played and Tibetan folk performers sang and danced.
Chinese troops entered Tibet in 1951, though Beijing claims the region has been part of Chinese territory for centuries. The Dalai Lama, then Tibet's supreme leader, fled to India after an abortive uprising against Chinese rule in 1959.