Xinhua reporters learnt that many policemen on duty were badly injured.
Police have not made any announcement of arrest, but an officer said the search for the vandals could be difficult as the mobs disguised themselves in plain clothes as ordinary citizens.
Around midnight, fire-fighters and policemen were cleaning the burning wreckages discarded on the Beijing Middle Road, one of the main streets in downtown Lhasa.
Police cordoned off a few downtown sections and are on close lookout for comeback of violence.
The regional government took emergency measures to rescue residents under attack, reinforced protection for schools, hospitals and gas stations, and required the government agencies and businesses to ensure safety of their employees.
Local government imposed traffic control on the main streets in Lhasa Friday night and it also informed the citizens of the sabotage through TV, calling for them to take precautions.
A blogger who called himself Han Jingshan, a Lhasa resident, recalled the sabotage in a post titled Four-hour Personal Experience of Lhasa's Riots.
The man drove a car onto the streets in the afternoon only to find flames with heavy smoke blanketing the area of the Ramogia Monastery and ambulances whistling by, according to the post.
"Arriving at the road entrance to the Ramogia Monastery, I saw the ground was covered with rocks weighing one or two kilograms and a cab was burnt down," he wrote.
"I saw a dozen of mobs, mainly young people in their twenties, were burning cars in front of the Baiyi Supermarket with more than 200 people standing by and watching," he said.
"At 17:56, a police car arrived and the mobs ran away. The police didn't chase them as another two cabs were on fire at the New Jiangsu Avenue some 300 meters ahead."
"At this time, a Han Chinese women, whose face was bleeding, ran by me and later a cab with broken windows passed by," he wrote.
"The history of Tibet and even of China will remember the day of March 14 in 2008 forever," the writer said.