Pelosi, one of the fiercest Congressional critics of China, was greeted by cheering Tibetans as she arrived to meet the Dalai Lama. She is the first major official to visit the leader of Tibet's exile community since peaceful protests turned violent last week in the Chinese-ruled region.
"If freedom loving people throughout the world do not speak out against China's oppression in China and Tibet, we have lost all moral authority to speak on behalf of human rights anywhere in the world," Pelosi said before a crowd of thousands of Tibetans, including monks and schoolchildren.
"The situation in Tibet is a challenge to the conscience of the world," she said.
Pelosi, heading a Congressional delegation, was greeted warmly by the Dalai Lama, who draped a gold scarf around her neck.
Hundreds of people lined the roads to the Dalai Lama's compound, some with signs saying "Thank You for Your Support" and "Long Live America-Tibet Friendship." About 2,000 more people waited in the temple's main courtyard, many waving Indian, U.S. and Tibetan flags.
The crowds roared when she arrived, applauding for about ten minutes.
"Today we are here at this sad time together in shedding the bright light of truth on what is happening inside Tibet," Pelosi said. "We insist the world know what the truth is inside Tibet."
Kalsing Phuntsok, 37, a teacher who was in the crowd welcoming Pelosi, called her "a very good friend of Tibet."
"America has a big role to play, a very big role," he said.
The visit was planned before protests against Chinese rule in the Tibetan capital of Lhasa erupted into rioting on March 14, sparking a brutal crackdown by Beijing.
Thousands of Chinese troops have converged in Tibetan areas of western China, ready to clamp down on any signs of further unrest.
"Perhaps it our karma, our fate, to be with you at such a sad time," Pelosi said. "It is our karma, we know, to help the people of Tibet."
The protests have been the biggest challenge in almost two decades to Chinese rule in Tibet, a Himalayan region that the People's Liberation Army occupied in 1950 after several decades of effective independence.