Tibet’s centuries-old yogurt festival has been marred this year by huge police presence and tight restrictions on Tibetans, sources from China’s western region said.

Yogurt occupies a special place in Tibetan culture and religion. Made from yak milk instead of cow milk, it has a history in Tibet going back thousands of years.

The festival, one of the most popular in Tibet, which runs from Aug. 16-22, began as an observance in the 17th century during which Tibetans offered yogurt to Buddhist monks who had completed their annual religious retreats.

Followers of Tibetan Buddhism viewed a 500-square-meter (5,382-square-meter) embroidered thangka portrait of the Buddha displayed at Drepung Monastery in Lhasa, capital of the Tibetan Autonomous Region, and later at Sera Monastery, the sources said.

But a large number of Chinese police watched the crowds and set up inspection booths along the streets in Lhasa to check devotees’ identity cards amid the celebrations, they said.

The heavy surveillance comes as Chinese authorities are maintaining a tight grip on Tibet, restricting Tibetans’ political activities and peaceful expression of cultural and religious identity.

Tibetans frequently complain of discrimination and human rights abuses by Chinese authorities and policies they say are aimed at eradicating their national and cultural identity.

Along with religious observances, the festival also features performances of traditional Tibetan opera, ethnic songs and dances, cultural exhibitions, and picnics.

Also, an official directive has barred government employees in Lhasa from attending religious activities related to the festival and taking part in any gatherings, and has ordered them to not engage in activities that disregard China’s communist government.

Government retirees are also barred from engaging in religious activities and visiting local monasteries. The directive states that they must educate their children about remaining loyal to the Chinese Communist Party and to value its socialist principles.