A fire broke out at the most important shrine in Tibetan Buddhism - the Jokhang Monastery in Lhasa.
Footage posted online showed flames billowing from the roof and what appeared to be at least one gutted pagoda.
Chinese state media said the blaze started late on Saturday and was soon put out. No casualties were reported.
The extent of damage to the sprawling compound is unclear. State media said there was no damage to cultural relics.
Tibet, the remote and mainly-Buddhist territory known as the "roof of the world", is governed as an autonomous region of China.
The Jokhang monastery is more than 1,000 years old and is listed as a Unesco World Heritage Site.
Reports say that Chinese authorities quickly tried to block footage and images of the fire appearing on social media.
Beijing maintains tight control of news reports from Tibet, where Buddhist sites have been a focal point for separatist unrest in the past.
The fire comes as Tibetans celebrate Losar, their traditional new year, which began on Friday.
Tibet has had a tumultuous history, during which it has spent some periods functioning as an independent entity and others ruled by powerful Chinese and Mongolian dynasties.
China sent in thousands of troops to enforce its claim on the region in 1950. Some areas became the Tibetan Autonomous Region and others were incorporated into neighbouring Chinese provinces.
China says Tibet has developed considerably under its rule.
But rights groups say China continues to violate human rights, accusing it of political and religious repression - something Beijing denies.