China declared martial law in the Tibetan capital of Lhasa Tuesday following three days of anti-Chinese rioting by pro-independence activists in which at least 12 people were killed.

The measure marked the first time since the Cultural Revolution that martial law has been publicly announced in China.Police slapped a temporary curfew and cleared the streets in the afternoon, threatening to shoot anyone found in the central square or in the Tibetan section nearby, an American witness said. They had fired on protesters earlier Tuesday, said foreign travelers.

The State Council, or cabinet, said martial rule would be imposed at midnight. The order was signed by Premier Li Peng, the official Xinhua News Agency said.

By early this afternoon, groups of 20 to 30 police armed with automatic weapons, shields and sticks had begun taking positions throughout the Tibetan part of Lhasa, telling all people to get off the streets, an American witness said.

"All the Tibetans just disappeared, everybody just evaporated," said the American who was given shelter in a Tibetan house. "All demonstrating stopped."

By early evening, Tibetans had ransacked a Chinese-owned store and started a bonfire on the city's main street.

The Xinhua report said the martial law decision was taken because "some separatists have been constantly causing disturbances in the city over the past few days." The nightly news said Lhasa city government has been authorized to take "any measures necessary to restore order."

Chinese authorities say 12 people were killed and more than 100 injured Sunday and Monday in clashes between police and Tibetans demanding an end to Chinese rule over the mountainous and remote southern region.

There were no reports of casualties Tuesday.

Western travelers quoted Tibetans as saying as many as 75 people died and 300 were wounded. There was no independent confirmation.