Hundreds of Tibetans in Lhasa have been detained by Chinese security officers after two self-immolation protests against Chinese rule over Tibet, a US-broadcaster said, stoking concerns of spreading unrest among Tibetans in China.

On Sunday, two Tibetan men set themselves on fire in Lhasa, state news agency Xinhua said, the first time in four years of a major Tibetan protest against Chinese rule. One of the men died.

China has branded the self-immolators "terrorists" and criminals and has blamed exiled Tibetans and the exiled Tibetan spiritual leader, the Dalai Lama, for inciting them.

At least 35 Tibetans have set themselves on fire since March 2011 in protest against China's six-decade rule over Tibet, according to Tibetan rights groups. At least 27 have died.

Late on Wednesday, Radio Free Asia cited a source as estimating that about 600 Tibetans had been detained since the Sunday's protests in Lhasa. The number could not be independently confirmed because foreign journalists are barred from entering Tibet.

Hao Peng, deputy party secretary in the Tibet Autonomous Region, has urged authorities to tighten their grip on the Internet and mobile text messaging, reflecting government fears about unrest during a month-long Buddhist festival which started last week.

The move is the latest in a series of measures the government says are intended to maintain stability.

"Hao Peng stressed that...the trouble caused by the activities of the Dalai clique has persisted, and the situation for stability maintenance is still complicated and grim," the official Tibet Daily newspaper reported.

The detentions come amid news that a Tibetan woman had set herself ablaze on Wednesday afternoon in Aba prefecture in southwestern Sichuan province, according to Tibetan advocacy group Free Tibet and Radio Free Asia.

Experts say Beijing may introduce tighter restrictions to halt the growing unrest in China's ethnic Tibetan areas.

Beijing considers the Dalai Lama, who fled into exile in India in 1959 after an abortive uprising against Chinese rule, a separatist. The Dalai Lama says he merely seeks

- Reuters