There were no reports of any casualties, state-run Xinhua news agency reported, without specifying the damage caused by the fire to the temple.
The fire started at 6.40 pm yesterday but was soon brought under control, it said.
Firefighters doused the fire and the relics are intact, state-run China Daily reported, without providing details.
The Jokhang temple, also known as the Qoikang Monastery, is regarded by the Tibetans as the most sacred and important temple in Tibet.
Internet users posted pictures and videos of the fire on social media, showing flames shooting high into the sky above old Lhasa.
The extent of the fire damage to the site, considered the spiritual heart of Tibetan Buddhism, was not known, the Post said.
The oldest part of the temple was built more than 1,300 years ago and the whole structure was added to UNESCOs World Heritage list in 2000 as part of the Potala Palace, the official home of successive Dalai Lamas.
The temple attracts thousands of pilgrims and tourists from home and abroad.
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