China has detained large numbers of Tibetans for political re-education after they returned from a visit to India to listen to religious teachings, a leading rights group said.

The New York-based Human Rights Watch (HRW) quoted multiple sources as saying that since February 6, many recently-returned Tibetans had been detained in ad hoc centres in Lhasa, capital of Tibet, and other areas.

Beijing has launched a huge clampdown on Tibetan-inhabited areas of China following several bouts of deadly unrest, and ahead of the March anniversary of the Tibetan spiritual leader the Dalai Lama's flight into exile.

The group said the exact number of detainees was not known, but may run into the hundreds. It is also unclear how long they will be held, but sources with knowledge of the detentions said they could last from 20 days to three months.

"This is the first known instance since the late 1970s in which the Chinese authorities have detained laypeople in Tibet in large numbers to force them to undergo re-education," HRW said in a statement received Friday.

The government and police of Tibet were not immediately available for comment when contacted by AFP.

Tibetans caught returning to China from Nepal or India without legal travel documents usually face stiff penalties, but the detention of travellers with valid passports is rare, the group said.

In the recent cases though, the detained returnees travelled in and out of China on valid passports and had entry visas for Nepal, it added.

A number of them also travelled directly to India using visas issued by India, indicating that Chinese authorities had not placed restrictions on travel to India in Tibetans' passports, as has been the case in the past.

China has imposed virtual martial law in parts of its vast Tibetan-inhabited regions as tensions have escalated, leading to the deaths last month of at least two people in clashes between police and locals in the southwest province of Sichuan.

Over the past year at least 20 Tibetans, many of them Buddhist monks, have set themselves on fire in protest at what they say is religious and cultural repression.

China accuses overseas groups seeking to split Tibet from the rest of China of fomenting the recent unrest, but rights groups say it stems from growing unhappiness among Tibetans over religious and cultural repression.

Foreign ministry spokesman Liu Weimin said he was not aware of the detentions but blamed "overseas personnel and organisations" for inciting the self-immolations and "other extreme activities and violence."