The Chinese Army drove through the streets of Lhasa Monday parading dozens of Tibetan prisoners in handcuffs, their heads bowed, as troops stepped up their hunt for the rioters in house-to-house searches.

Shortly before the midnight deadline passed for rioters to surrender, four trucks in convoy made a slow progress along main roads, with about 40 people, mostly young Tibetan men and women, standing with their wrists handcuffed behind their backs, witnesses said.

A soldier stood behind each prisoner, hands on the back of their necks to ensure their heads were bowed.

After the clock struck midnight, more than 100 demonstrators gathered outside the Chinese embassy in London, according to Agence France-Presse.

The protesters chanted "Free Tibet, stop the killing in Tibet!" and "Chinese out!", according to AFP, which reported that the throng charged in the direction of the embassy before being stopped by police.

During the roundup earlier Monday night in Tibet, loudspeakers on the trucks broadcast calls to anyone who had taken part in the violent riots on Friday — in which Han Chinese and Hui Muslims were stabbed and beaten and shops and business set on fire — to turn themselves in. Those who gave themselves up might be treated with leniency, the rest would face severe punishment, the broadcasts said.

China accused Tibetan supporters of the Dalai Lama of attacking its embassies around the world, vowing Monday to protect its territory in the central government's first comments on violent protests against Chinese rule of Tibet, according to The Associated Press.

The statement came as more clashes erupted in other Chinese provinces and a midnight deadline passed for protesters in Tibet's capital to surrender or face harsh consequences. Residents of Lhasa feared a military sweep after midnight, the Washington-based International Campaign for Tibet said.

"The Chinese government will unwaveringly protect national sovereignty and territorial integrity," Foreign Ministry spokesman Liu Jianchao said.

Protests that began in Tibet have spilled over to neighboring provinces and even to the capital, Beijing, where students staged a sit-down demonstration Monday. There have been sympathy protests around the world as well, many of them outside Chinese diplomatic missions.

The worst violence in 20 years in the deeply Buddhist Himalayan region has drawn a tough response from the Government, facing severe embarrassment as the riots threaten to tarnish its image of unity and stability only five months before it plays host to the Olympic Games in Beijing.

Claims and counterclaims from Chinese officials and Tibetan exiles over the number of casualties and a ban on foreign journalists in Tibet have resulted in much confusion.

Champa Phuntsok, the ethnic Tibetan governor of the Tibet Autonomous Region, said the demonstrations had left 16 dead and dozens wounded. Unconfirmed reports from Tibetan exile groups put the death toll at 80 — a claim he denied.

The governor said: “No country would allow those offenders or criminals to escape the arm of justice and China is no exception.”

Speaking in Beijing, where he is attending the annual session of China’s rubber-stamp parliament, the governor said that shops, schools, hospitals and banks had been attacked and bystanders beaten and set on fire.

“If these people turn themselves in, they will be treated with leniency within the framework of the law. If these people could provide further information about the involvement of other people in those crimes, then they could be treated even more leniently . . . [otherwise] we will deal with them harshly.”

The search for those involved began in earnest in Lhasa Monday, as office workers trickled back to work after a weekend of fear when most dared not go outside.

Soldiers began house-to-house searches, checking all identification papers, residents said. Anyone unable to show an identity card and a household registration permitting residence in Lhasa was being taken away.

They described people laying out all their papers on a table in their homes. One said: “The soldiers come in and check that the number of people in each house equals the number of identity cards. Anyone extra may be taken away.”

At government offices and work units, leaders were required to do a roll call of all employees and to account for anyone missing, as the authorities tried to track down those involved in the violence.

A notice from the Lhasa Municipal Procuratorate said: “Criminals who do not surrender by the deadline will be treated severely in accordance with the law.”

The unrest has spilt over rapidly into neighboring provinces in China with a large ethnic Tibetan population. Tibetan students at the NorthWest Minorities University in Lanzhou staged an all-night sit-in at a school sports field before dispersing this morning.

In the nearby town of Hezuo, in northwestern Gansu province, several dozen students from the Hezuo School of Hygiene took to the streets to demonstrate in sympathy for Tibetans in Lhasa but were quickly dispersed by police, school officials said.

The unrest in Tibet began when monks took to the streets on the March 10 anniversary of a 1959 uprising against Chinese rule, when the Dalai Lama fled into exile in India with tens of thousands Tibetans.

Beijing has repeatedly said that the violence was engineered by supporters of the Dalai Lama. He is still the region's widely revered spiritual leader and one of the figures most reviled by China’s communist leadership.

The Associated Press contributed to this report.