BEIJING — The surprising escape of a blind legal activist from house arrest to the presumed custody of U.S. diplomats is buoying China's embattled dissident community even as the government lashes out, detaining those who helped him and squelching mention of his name on the Internet.

The flight of Chen Guangcheng, a campaigner for disabled rights and against coercive family planning, is a challenge for China's authoritarian government and, if it's confirmed he is in U.S. custody, for Washington too. Assistant Secretary of State Kurt Campbell began a hurried mission to Beijing on Sunday to smooth the way for annual talks involving his boss, Hillary Clinton, Treasury Secretary Timothy Geithner and scores of officials.

Though Chen — a self-taught legal activist described by friends and supporters as calm and charismatic — hardly seems a threat, security forces and officials have reacted angrily, detaining several of his supporters and a nephew who fought with officials after the escape was discovered is on the run.