"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 cheering Tibetans, including monks and schoolchildren.
After meeting with the Tibetan leader, Pelosi called for an international investigation into the violence in Tibet, and said the Chinese should open the region to the international media and independent monitors.
She dismissed China's accusations that the Dalai Lama was behind the violence, saying they made "no sense".
James Sensenbrenner, house representative of Wisconsin, the ranking Republican on the trip, also expressed his support for the Tibetan people.
"In the US Congress, there is no division between Democrats and Republicans on the issue of protecting Tibetan culture and eliminating repression against Tibetans around the world," he said.
|Chinese military trucks line the streets where|
the deadly protests have been raging [AFP]
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, US and Tibetan flags.
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 turned violent last week in the Tibetan capital, Lhasa, sparking a crackdown by Beijing.
Pelosi said: "Perhaps it's our karma, our fate, to be with you at such a sad time. It is our karma, we know, to help the people of Tibet.
"Today we are here at this sad time together in shedding the bright light of truth on what is happening inside Tibet. We insist the world know what the truth is inside Tibet."
Thousands of Chinese troops have converged in Tibetan areas of western China, ready to clamp down on any signs of further unrest.
"Today we are here at this sad time together in shedding the bright light of truth on what is happening inside Tibet"
Nancy Pelosi, speaker of US House of Representatives
A spokesman for Tibet's government-in-exile said 19 Tibetans had died in China's Gansu province in recent days following anti-government protests, raising the total death toll in the region to 99.
Thuten Samphel provided no details on how the 19 people died. China says 16 people have been killed since anti-government protests turned violent in the Tibetan capital last week.
Chinese officials have accused the Dalai Lama and his supporters of organising violent clashes in Tibet in the hope of sabotaging this summer's Beijing Olympics and promoting Tibetan independence.
The protests have been the biggest challenge in almost two decades to Chinese rule in Tibet.