Chinese authorities have deployed police and soldiers throughout Tibet on the 50th anniversary of an uprising against Chinese rule.
Chinese President Hu Jintao urged Tibet's leaders to ensure lasting order and social stability. Chinese authorities want to prevent a repeat of last year's protests on the sensitive March 10 anniversary.
Chinese officials on Monday also confirmed a security buildup along Tibet's border with the Himalayan regions of India and Nepal.
In Washington, hundreds of Tibetan exiles joined by Chinese dissidents and other supporters marched Monday from the White House to the Chinese embassy.
On Capitol Hill, House Speaker Nancy Pelosi told lawmakers gathered for a commemoration of the uprising that the human rights situation in Tibet has deteriorated over the last year.
She said the United States will not forget the Tibetan people, who she said have accumulated legitimate grievances from decades of repressive Chinese policies.
U.S. lawmakers will vote this week on a nonbinding resolution that would call for a multilateral effort to bring about a peaceful solution to the Tibet issue.
The Associated Press reports that Chinese authorities on Monday closed the last area of Sichuan province's Ganzi Tibetan Autonomous Prefecture to foreigners. An AP reporter said police with machine guns and riot gear marched in the streets of the city of Kangding, Tibetan: Dartsedo.
Earlier Monday, Chinese state media reported a protest in a Tibetan area of Qinghai province. The reports said police vehicles and a fire truck were set on fire by handmade explosives during the incident. No casualties were reported.
Tibetans in Sichuan and Qinghai joined demonstrations last year that spread across the Tibetan plateau after monk-led protests in Tibet's capital, Lhasa, turned violent.
China reported that about 20 people were killed by protesters during violence in Lhasa on March 14, 2008. But Tibetan exiles and human rights groups say the number of Tibetans who died during the crackdown in Tibet and neighboring provinces was about 200.
The U.S.-based International Campaign for Tibet on Monday said about 1,200 Tibetans remain missing.
Tibetan exiles and their supporters are planning to hold protests across North America, Europe and Asia on Tuesday to mark the anniversary.
Tibet's exiled spiritual leader, the Dalai Lama, will address a gathering in Dharamsala, the north Indian town that has been his home since he fled Tibet in 1959.
China's central government contends the Buddhist leader has incited activists to campaign for Tibet's independence. The Dalai Lama says his only goal is to achieve genuine autonomy for Tibetans within China.
Some information for this report was provided by AFP, AP, Bloomberg and Reuters.