Agence France-Presse

BEIJING - China's deadline passed early Tuesday for protesters to hand themselves in to authorities in Tibet, which remained cut off from the outside world after a lockdown by Chinese security forces.

Beijing had ordered anyone involved with last week's unrest, the biggest demonstrations in Tibet against Chinese rule in 20 years, to surrender overnight or face what it called "serious consequences".

There was no immediate word from officials if anyone had done so, and even activist groups with long-standing connections in Tibet indicated they were having difficulty finding out what was happening in the region.

"It is a very, very tense and terrifying situation," Kate Saunders, from the International Campaign for Tibet, an activist group, told AFP. "But it has become much more difficult to get information out."

Just months before China hosts the Olympic Games, Friday's turbulence -- which left 13 dead according to China, around 100 according to Tibetan exile groups -- has renewed international attention on China's human rights record.

While there have been few calls for any boycott of the Games in August, China has flatly rejected any criticism of its handling of the protests and has blamed Tibetan "mobs" for killing what it said were innocent people.

Premier Wen Jiabao said Tuesday that China had evidence that groups aligned with Tibet's exiled spiritual leader, the Dalai Lama, were responsible for the unrest.

The Dalai Lama has denied being behind the violence and has denounced what he called China's "cultural genocide" and "rule of terror" in the region.

Rioters targeted Chinese shops and banks in remote Tibet, which China officially calls an "autonomous region" but where there has been resentment over Chinese rule ever since China annexed the area decades ago.

Foreign tourists and journalists have been blocked from the region since Friday's events, and witnesses say the Tibetan capital Lhasa is under virtual lockdown, with Chinese tanks and soldiers reported on the streets.

"Shops remain closed and people are said to be surviving on what little provisions they might have at home," Tibetinfonet, an on-line information service, said in a statement.

China said that Tibetan rioters murdered 13 innocent civilians in the riots and denied using any deadly force to quell the uprising.

"Throughout the process, (security forces) did not carry or use any lethal weapons," China's top official in Tibet, regional government chairman Qiangba Puncog, said on Monday.

But exiled Tibetan leaders said on Monday that about 100 people, and possibly hundreds, were killed in the crackdown on demonstrators.

Qiangba said anyone who had committed serious crimes would be dealt with harshly but that the government would be lenient with those Tibetans who informed on demonstrators.

Among international officials, UN Secretary General Ban Ki-moon urged China to show restraint while US Secretary of State Condoleezza Rice called on Beijing to open talks with the Dalai Lama.

The Buddhist leader has lived in exile ever since 1959, when he fled following a failed uprising in Tibet against Chinese rule in the Himalayan region.

Wen acknowledged on Tuesday that protests had spread outside of Tibet to other parts of China.