Police and troops locked down the capital of Tibet on Sunday as tension remained high, two days after ugly street protests against Chinese rule that some sources claimed had killed dozens of people.

"We don't dare go out, not for anything. There's too much trouble," said a Tibetan businesswoman by telephone from Lhasa, a remote city high in the Himalayas out of bounds to foreign journalists without official permission.

Like other residents contacted, she spoke only briefly and anonymously out of fear of punishment in a city that last week saw the most serious rioting and shooting in nearly two decades.

The convulsion of Tibetan anger at the Chinese presence in the region came after days of peaceful protests by monks and was a sharp blow to Beijing's preparations for the Olympic Games in August, when China wants to showcase prosperity and unity.

China has declared a "people's war" of security and propaganda against support for the Dalai Lama, the spiritual leader of Tibetan Buddhists, underlining that it will not heed calls from around the globe for a lenient response to the riots.

US Secretary of State Condoleezza Rice on Saturday voiced strong concern over the authorities' response to the protests.

"I am concerned that the violence appears to be continuing. I also am concerned by reports of a sharply increased police and military presence in and around Lhasa," she said in a statement released in Washington.

European nations and Australia have also urged China to find a peaceful outcome, and Japan has expressed concern.

A woman in contact with a businessman in Lhasa said the streets were teeming with armed police in riot gear on Sunday after word of renewed clashes the previous night, when Hui Muslim Chinese attacked Tibetans in revenge for wrecked homes and property.

"The Tibetans were starting to fight back but then the troops stepped in and restored order," she said. The report of fresh fighting could not be verified.

China has said at least 10 "innocent civilians" died, mostly in fires lit by rioters. On Sunday the official Xinhua news agency said 12 policemen had also been seriously injured. "The protesters were barbarous and violent," Xinhua quoted a tourist surnamed Dong as saying. "They ganged up on the young police officers and beat innocent people."    

The self-proclaimed Tibetan government-in-exile in northern India has said some 30 people were killed in clashes with Chinese authorities. But a member of the Tibetan parliament in exile who lives in Taipei, Khedroob Thondup, said even that was far below the real number and cited a contact who claimed to have counted many more corpses of people killed in the riots or subsequent crackdown.

"He said there were 67 bodies in one morgue alone," Thondup told Reuters. "He saw it with his own eyes."    

Last Friday Tibetan protesters, some in Buddhist monks' robes and some yelling pro-independence slogans, trashed shops, attacked banks and government offices and wielded stones and knives against police. The monks had taken to the streets on Monday to mark the 49th anniversary of an earlier uprising. The protest later spread to neighbouring provinces inhabited by pockets of Tibetans. Xiahe in Gansu province saw hundreds of monks and lay residents march in peaceful defiance, to judge from pictures sent to reporters.

A witness who was in Xiahe early on Sunday said the Tibetan quarter of town was sealed off by anti-riot troops, and on the Chinese side of town there was evidence of recent violence.

"The whole glass front of this hotel was smashed and you could see a lot of windows broken all along the street," the witness said.

Chinese authorities have now signalled a sweeping campaign to redouble security in the region and attack public support for the Dalai Lama, who fled over the Himalayas into exile in 1959