Close to 1,000 Tibetans have been detained in two days of sweeps across Lhasa, the capital, by paramilitary police hunting down those who took part in last week’s deadly anti-Chinese riots.

Sources in the city said 600 people had been detained on Saturday and another 300 had been picked up on Sunday. They said it was not clear where those rounded up were being detained because the main Drapchi prison in Lhasa is believed to be virtually full.

Those detained could be taken to the old Number One prison in the Sangyip district in the northeast of Lhasa that is not currently believed to be in use. They may be held in the nearby Number Four detention centre and the New Lhasa prison in the same district that has recently been used as a re-education-through-labour centre. They could even be taken to the new Chushur prison some distance outside Lhasa where most political prisoners are believed to be jailed after sentencing.

Chinese officials were not available to confirm the total number of arrests.

With the expiry at midnight yesterday of a deadline for the Tibetan protesters who on Friday stabbed and hacked ethnic Han Chinese, hurled rocks and set fire to offices, shops and schools, the search for those involved has gathered momentum.

In one residential area of Lhasa near the Potala Palace, a group of police and paramilitary officers raided one apartment block late in the afternoon, drawing a huge crowd of onlookers. They dragged a man of about 50 out of the building along with two young men. All appeared to be city residents and not peasants or nomads from Tibet's vast grasslands.

The paramilitary officers then fired three rounds of tear gas to disperse the crowd that had swollen rapidly during the operation, witnesses said. Murmurs of anger had begun to reverberate among the passersby as police beat the older man, believed to be an ethnic Tibetan Government cadre, as they dragged him from the building and into a waiting security vehicle.

In the Karma Lunsang district, a warren of old Tibetan homes in the east of the city that the authorities suspect has served as an important hideout for the protesters, police and paramilitary were going house to house to check identity papers. One witness said: “Many people have been taken away, but we don’t know how many.”

The sources said it was not known how many people might have surrendered in return for promises of leniency before the midnight deadline or how many had been arrested since Monday.

Police and troops were manning checkpoints across the city, checking all identity papers and it was still quite difficult for people to move easily through the streets.

Foreign journalists travelling in areas near Tibet have reported movements of troops in the direction of the Himalayan region. Sources said garrisons of the People’s Liberation Army around Lhasa have been placed on a grade one alert in case of more trouble.

Chinese authorities have blamed Tibetan mobs manipulated by the exiled Dalai Lama for the deaths of 13 people in the riots on Friday. Tibetan exile groups have said that as many as 100 people may have been killed as troops backed by armoured personnel carriers moved into the city to quash the biggest protests against Chinese rule in 19 years.

Wen Jiabao, the Chinese Premier, in his first remarks on the unrest, accused the rioters of trying to disrupt the Olympic Games that start in Beijing on August 8. “They wanted to incite the sabotage of the Olympic Games in order to achieve their unspeakable goal."

Although Lhasa was quiet, the unrest has spread to nearby provinces with large Tibetan populations. In northwestern Gansu, which borders Tibet, large numbers of ethnic Tibetans took to the streets late on Sunday, burning shops and business belonging to ethnic Han Chinese and Hui Muslims and burning 16 cars, said one witness.

From Monday night all government offices had been ordered to remain on duty around the clock. A local government order said: “Without a notice, no one may leave their posts.”

In neighbouring Sichuan province, an ethnic Tibetan told Reuters he knew of no fresh outbreaks of unrest since Monday. “Now they are bringing back stability. There are so many police and People's Armed Police it will be difficult for anything to spread. I'm sure the People's Liberation Army is waiting, too. In the background waiting, if the situation really gets out of hand."