Reports from Tibet say Chinese security forces have launched a wave of detentions in Lhasa following two self-immolations there on Sunday.
Witnesses to the immolations had been arrested and Tibetans from outside Lhasa sent home, the reports said.
The two Tibetan men set themselves on fire outside the Jokhang temple in an apparent anti-China protest.
There have been several such incidents in Tibetan areas outside Tibet but these were the first in Lhasa.
One of the men died while the other "survived with injuries", Xinhua news agency said. Both were reportedly Tibetans from outside Tibet.
Meanwhile a Tibetan mother of three has died after setting herself on fire in Sichuan province, reports say.
The woman, in her 30s, set herself on fire outside a monastery in Aba county, an ethnically Tibetan area where many of the self-immolations have taken place.
US broadcaster Radio Free Asia reported that an estimated 600 Tibetans had been rounded up in Lhasa since the incident on Sunday, citing unidentified sources.
Foreign media are banned from the area, making reports difficult to verify. Tibetan groups with contacts in Lhasa say that number cannot yet be confirmed.
But Thubten Samphel, a spokesman for the Tibetan government-in-exile in Dharamsala, told the BBC that he had also spoken to people with contacts in Lhasa who said arrests were taking place.
"We have been told that Tibetans who do not belong to Lhasa have been expelled to their native places," he said.
Tsering Tsomo, of the Tibetan Centre for Human Rights and Democracy in Dharamsala, said her organisation had heard about arrests of people who had witnessed the self-immolations on Sunday.
She said the temple had now been closed to foreigners and pilgrims, and the site of the self-immolations condoned off.
The BBC contacted three government departments in Lhasa, but they either did not pick up the phone or said they knew nothing about the arrests.
More than 30 Tibetans - mostly young monks and nuns - have set themselves on fire since March 2011. Many of them are reported to have died while the condition of some remains unknown.
Most of the incidents have take place in ethnically Tibetan areas outside Tibet.
China's leaders blame the Dalai Lama, the Tibetans' exiled spiritual leader, for inciting the self-immolations and encouraging separatism.
He rejects this, and both activist groups and the Tibetan government-in-exile say the self-immolations are protests against tight Chinese control of the region and religious repression.
The woman who set herself on fire in Aba county on Wednesday was identified as a mother of two daughters and a son. She died at the scene, the reports said.