Tibet's Government-in-exile says the United Nations has proved itself to be a "white elephant" by failing to act more decisively over China's repression of protesters.

The criticism came amid reports that Chinese forces had been ordered to shoot Tibetan protesters on sight after a midnight deadline to surrender passed.

Riot police and paramilitary forces are keeping a tight grip on Tibetan communities across western China after the deadline for protesters in the Tibetan capital Lhasa to give themselves up to authorities expired.

Government-in-exile Prime Minister Sampdong Rimpoche accused the Chinese Government of "brutal behaviour".

He said there was no international agency to prevent the shoot-to-kill orders being carried out and described the United Nations as an undemocratic body.

"The international community are also a silent onlooker. There has been no concrete action except consoling words," he said.

"The UN system in my view is the most undemocratic institution in this world. It's just a white elephant."

The Government-in-exile now claims around 100 people have died in the Chinese crackdown.

There are no reports at this stage of demonstrators handing themselves in.

United Nations Secretary General Ban Ki-moon called for restraint from China after the deadline passed.

"At this time I urge restraint on the part of the authorities and call on all concerned to avoid further confrontation and violence," he said after meeting China's ambassador to the UN.

China is a permanent member of the UN Security Council with the power to veto any attempts to act on Tibet.

Protests continue

Protests continued overnight in the Chinese provinces of Gansu, Qinghai and Sichuan, as well as in Beijing and in the Nepalese capital Kathmandu.

In Gansu, Tibetan students have staged sit-ins, while in Sichuan there have been more bloody clashes reported between demonstrators and riot police.

Fresh protests flared near two Sichuan Tibetan schools, with hundreds of students facing police and troops and about 40 students from a high school were beaten and arrested.

Monasteries in Qinghai are all under close surveillance, but many Tibetans remain determined to seize this opportunity to be heard.

Ethnic Tibetan students have staged a candle-lit vigil in Beijing to pray for the dead.

Police kept reporters well away from the peaceful protest by dozens of students inside the Central University for Nationalities.

It was a small, rare show of defiance in the host city of this year's Olympic Games, where Communist Party authorities are eager to prevent public shows of dissent.

The vigil was broken up by authorities just hours before Lhasa's deadline.

The fact that Tibetans are prepared to demonstrate, despite the huge security presence is perhaps a measure of the depth of their frustration.

They know the international community is watching them, but it is becoming more difficult to get their message out.

Foreign journalists are now facing heavy restrictions. Many have been ordered to leave China's restive west and some have been detained for visiting monasteries where protests have taken place.

Lhasa locked down

Meanwhile, heavy security remained in place on the streets of Lhasa after the deadline passed.

The city has been quiet for the past 24 hours and there has been no repeat of the anti-Chinese riots of recent days.

The Tibetan government in exile in India says protests by Tibetans have continued elsewhere in China, some involving thousands of people.

It says house-to-house searches have been going on and a number of former political prisoners are reported to have been detained again.

Meanwhile the Chinese Government has maintained its attack on the Dalai Lama, claiming to have proof that he organised the riots in Tibet.

"Regarding this entire incident ...we have enough evidence to prove that these actions were completely organised by the Dalai Lama," Chinese Government spokesman Liu Jianchao said.

He did not produce any of evidence but said the Chinese Government would divulge it at the appropriate time.