Hundreds of people have been arrested across Tibet in a Chinese crackdown following deadly riots.
Human Rights Watch said it had unconfirmed reports of hundreds of arrests and warned those in custody were in great risk of being tortured.
Officials with the public security bureau in Lhasa, Tibet, would not comment.
Journalists have been prevented from getting access to any of the areas where riots and violence have been reported.
Pro-Tibet activists have been bombarded with abusive phone calls and virus emails as they try to contact witnesses in Tibet and nearby amid the clampdown.
Matt Whitticase, from the Free Tibet Campaign, said he had received calls every two minutes from 4am to 7am yesterday to his mobile number and his work number in London.
‘Of course I have no way of saying who the calls were from, but a variety of callers (from British mobile numbers) had Chinese accents,’ he said.
‘The content was crude, abusive and highly anti-Tibetan in nature. The calls also contained the sort of patriotic Chinese music you used to hear on Chinese trains and in public places.
‘It seemed that the intention was to stop me from working and from making calls.’
Lhadon Tethong, director of Students for a Free Tibet, said the organisation's New York office has also received abusive calls from people speaking Chinese, and added that they had received viruses via email.
‘We are getting virus attacks that are just shameless...claiming to be desperate people inside Tibet. The emails are well-written and emotional, pleading for us to open the images,’ she said.
Tashi Choephel, a researcher at the India-based Tibetan Centre for Human Rights and Democracy, said the centre's email system was unusable because of attacks.
The centre had also received an anonymous phone call from Hong Kong trying to find out the source of photos it had published apparently showing Tibetan protesters with gunshot wounds and other severe injuries.
The groups are trying to contact people inside Tibet and the surrounding regions to find witness accounts of deadly violence that has erupted there in the past few days amid anti-Chinese protests.
But journalists, rights groups and activists are being denied access to almost every part of China where protests are reported to have taken place.
