The case also poses a major new diplomatic test for the United States. In February, the Obama administration was thrust into an internal Chinese political dispute when Wang Lijun, the former top police official from the region of Chongqing, sought refuge in the American Consulate in Chengdu. Mr. Wang revealed details about the killing of a British businessman, setting off a cascade of events that led to the downfall of Bo Xilai, who was the party chief in Chongqing and a member of China’s Politburo. American diplomats said they had determined that Mr. Wang’s case did not involve national security, and he was turned over to Chinese officials, prompting criticism from some in Washington about their handling of the case. Both sides insist Mr. Wang left of his own accord.

But with Mr. Chen now believed to be on the grounds of the American Embassy in Beijing, administration officials are likely to be far more cautious in handling his case. His advocacy for the handicapped and for families subject to forced abortions and other coercive population control methods is widely known in the West. He also became a symbol of the deficiencies of China’s legal system after he was convicted of criminal charges in 2006 in a prosecution that Chinese lawyers — and even some officials in Beijing — felt made a mockery of China’s claims to be developing better legal norms.

Mr. Chen, according to those who have spoken to him, slipped away on Sunday evening from his home in Shandong Province, where he has been held incommunicado since his release from prison in September 2010. Ai Weiwei, the artist and government critic who has also been subjected to residential detention, though far less draconian, said he had spoken to a friend who met with Mr. Chen in Beijing on Wednesday. The friend said Mr. Chen had climbed over a wall at night and evaded multiple lines of guards.

“You know he’s blind, so the night to him is nothing,” Mr. Ai said the friend told him. “I think that’s a perfect metaphor.”