Hundreds of people have been detained in Lhasa after two men set themselves on fire in the Tibetan regional capital, a US-based broadcaster said, as a young mother became the latest Tibetan to self-immolate.

Radio Free Asia said Chinese security forces had rounded up hundreds of residents and pilgrims in the wake of Sunday's incident, the first major protest in the heavily-guarded city since deadly anti-government riots in 2008.

It quoted a local source as saying about 600 Tibetans had been detained and those from outside the Tibet Autonomous Region (TAR) had been expelled.

Foreign journalists cannot go to the TAR without special permission and the report could not be independently confirmed. A security official in Lhasa said by telephone that she did not know anything about the reported detentions.

On Wednesday, a Tibetan mother of three self-immolated in front of a monastery in southwestern Sichuan province's Aba county, where many of the protests have taken place, Free Tibet and Radio Free Asia reported.

Free Tibet, a London-based campaign group, said the woman was in her mid-30s and died at the scene.

More than 30 people have set themselves on fire in Tibetan-inhabited areas of China since the start of March 2011 in protest at what they say is religious and cultural repression by the Chinese authorities.

Sunday's protest took place as Lhasa was filled with Tibetan Buddhist pilgrims who had travelled to the city to celebrate Saga Dawa - the anniversary of Buddha's birth.

The two Tibetan men, who were both from outside the TAR, set themselves on fire in front of the famed Jokhang Temple, a popular pilgrimage destination in the centre of the city.

Police immediately put out the flames and one of the two men survived, according to state news agency Xinhua. His current whereabouts are not known.

Sunday's incident was the first of its kind in the Tibetan capital, which has been under tight security since the 2008 riots.

Tibetans have long chafed under China's rule over the vast Tibetan plateau, saying that Beijing has curbed religious freedoms and their culture is being eroded by an influx of Han Chinese, the country's main ethnic group.

Beijing insists that Tibetans enjoy religious freedom and have benefited from improved living standards brought on by China's economic expansion.

On Thursday, the state-run Tibet Daily reported that a senior Communist Party official had ordered authorities to crack down on 'criminal activities' in Lhasa and on rumours spread via mobile phone and the Internet.

Residents of Lhasa said the city was under even tighter security than usual following Sunday's protest, with police and paramilitary officers out in force, although the streets were calm.

Tibet's government-in-exile said earlier this week it was 'seriously concerned' by the latest development and that the situation in Lhasa remained tense.

Tibet's exiled spiritual leader the Dalai Lama announced last year that he was giving up his political role and would focus on spiritual duties.

Nonetheless, Beijing has repeatedly accused him of trying to split Tibet from the rest of China and encouraging Tibetan protesters in the vast Himalayan region to set fire to themselves - a charge he denies.