the 50th anniversary of the failed uprising against Chinese rule and the flight of the Dalai Lama into exile.
The 150 activists, many dressed in native red, grey, purple and green silk costumes, stopped singing and talking for two minutes at the stroke of noon to commemorate the deaths of thousands of resistance fighters and monks over the decades.
Ngawang Sandrol, a nun who was imprisoned at age 13 and released 11 years later in 2003 under extreme international pressure, told the crowd how she was beaten on a daily basis by Chinese police using a variety of weapons. Her crime had been chanting two slogans: "Free Tibet" and "Long live the Dalai Lama", she said.
Electro-shock torture started when "I was still only 13 years old", she said.
"China rules Tibet with brutal occupation," she said.
While activists demonstrated outside the White House, another 150 supporters from 26 US states fanned out on Capitol Hill to buttonhole elected representatives about their cause. Organisers said it was the first-ever national lobbying day for a free Tibet.
Leaders of Tibetan activist groups in the US said they they were shocked by US Secretary of State Hillary Clinton's recent comments that human rights and Tibetan issues could not be allowed to "interfere" with the Washington-Beijing dialogue on three other crises - the global economic downturn, climate change and security issues.
"By releasing that statement, she gave China the green light" to continue its oppression, Tsering Palden, 36, a computer programmer who has lived in the US since he emigrated with his parents in 1983, told DPA.
The Tibetan activist community expects something different from Obama after his election promise of "change," said Palden, who was dressed in a grey traditional robe. He is president of the New York and New Jersey chapter of the Tibetan Youth Congress.
"We are pushing the president to lead a multilateral pressure campaign against China," Palden said. "Most governments are supportive but they are waiting for someone (like the US) to take the lead."
In her remarks to reporters, Clinton said that the question of Tibet and human rights was still "part of the agenda". But she added she didn't expect much to change.
"We pretty much know what they're going to say ... because I've had those conversations for more than a decade with Chinese leaders," she said.