A man who answered the telephone at the Yarlung Tsangpo Hotel in Lhasa said security in the city had been tightened and additional security forces sent in. The man, who gave his name as Mr. Liu, said it was unclear whether the new forces were made up of regular police officers or were units of the People’s Armed Police, a paramilitary force that is usually called out to quell riots and maintain security in the restive ethnic regions of western China.

Robert J. Barnett, a scholar of modern Tibet at Columbia University, said a Tibetan in Lhasa had told him the city was in a “boiling situation” after the self-immolations.

“We’re now seeing self-immolations that seem to be political expressions that are in sympathy with the core incidents that happened earlier,” Mr. Barnett said.

“The Chinese officials are really worried,” he said, because the latest protests seem to be “driven by an idea, a political goal.”

By contrast, he said, the earlier self-immolations in Ngaba were largely in reaction to security clampdowns at the Kirti Monastery after the 2008 uprising.

Ngaba has been the center of the self-immolations, but Tibetans have now set themselves on fire in areas across the vast Tibetan plateau. Most have been members of the clergy. Before the self-immolations in Lhasa, there had been just one such protest in the Tibet Autonomous Region, by a layman in the eastern area known as Chamdo.

The self-immolations on Sunday were first reported by Radio Free Asia and Voice of America, which have contact with Tibetans in western China. Voice of America reported that the two men worked at a restaurant in Lhasa called Nyima Ling. Radio Free Asia said the two were monks who were taken away in security vehicles within 15 minutes of setting themselves on fire.

In March, President Hu Jintao of China told the Tibet delegates to the National People’s Congress in Beijing that they must exert a “continuous effort in sustaining social harmony and stability.” Official news reports say Chen Quanguo, the current party chief of Tibet, repeated Mr. Hu’s words in public meetings and said officials would “persist in the thought that stability overrides all.”