BEIJING  In the chaotic hours after Lhasa erupted March 14, Tibetans rampaged through the city’s old quarter, waving steel scabbards and burning or looting Chinese shops. Clothes, souvenirs and other tourist trinkets were dumped outside and set afire as thick gray smoke darkened the midday sky. Tibetan fury, uncorked, boiled over.

Foreigners and Lhasa residents who witnessed the violence were stunned by what they saw, and by what they did not see: the police. Riot police officers fled after an initial skirmish and then were often nowhere to be found. Some Chinese shopkeepers begged for protection.

“The whole day I didn’t see a single police officer or soldier,” said an American woman who spent hours navigating the riot scene. “The Tibetans were just running free.”

Lhasa is now occupied by thousands of paramilitary police officers and troops of the People’s Liberation Army. But witnesses say that for almost 24 hours, the paramilitary police seemed unexpectedly paralyzed or unprepared, despite days of rising tensions with Tibetan monks.