BEIJING — Fires and rock-throwing protests erupted in the Tibetan capital of Lhasa on Friday as unrest against Chinese rule of the autonomous region escalated to its highest level in two decades.

Witnesses reported random gunfire in the city and said angry Tibetans were chasing down and beating Chinese in the streets.

The U.S. Embassy warned Americans to stay away from Tibet, noting “firsthand reports from American citizens in the city who report gunfire and other indications of violence.”

A flagship building in the old city of Lhasa, the Tromzikhang market, was heavily damaged in a fire, witnesses said. Protesters set tires afire in streets at various parts of the city, especially along Beijing Street. Witnesses said they had seen police cars and fire trucks that had been smashed and set afire.

A lockdown of three monasteries in the Lhasa area remained in effect to block saffron-robed monks from returning to the streets, as they did Monday and Tuesday, in an unusual display of opposition to Chinese rule, which has been marked by alternating periods of repression and restraint since Chinese troops seized control in 1950. After serious unrest in 1989, martial law was declared in Tibet.

“Lhasa reels under (an) extremely tense situation at the moment. . . . There is smoke every where from burning shops and vehicles,” according to a statement from the Tibetan Center for Human Rights and Democracy, a nonprofit advocacy group based in Dharamsala, India.

Internet accounts painted a picture of chaos in the city.

“People were just running randomly about. Some of them were looting,” said a witness who used the internet name “North Wind” on the fanfou.com website.

He said some 1,000 soldiers with riot shields had gathered in front of a stadium in Lhasa.