Yao Lifa, a well-known advocate of free elections from Hubei province in central China, was due to attend the reception held by the U.S. consulate in Wuhan, the provincial capital.

But Yao disappeared on Sunday in Beijing, failing to turn up at a meeting with friends there, his son Yao Yao told Reuters. Yao Lifa also failed to use a train ticket he had bought to take him to Wuhan on Monday.

Yao Yao said his father appears to have been detained by local authorities but also said he has no idea of his father's whereabouts. Yao Lifa's mobile phone had a ring tone when called, but nobody answered repeated calls.

"He's often been informally detained by officials from Qianjiang before, and I think it's the same this time," said Yao Yao, the son. "I think they did not like him going to the Obama inauguration reception."

In a phone conversation with a Reuters reporter last month, Yao Lifa said he felt inspired by the election of Obama. He had earlier visited the United States on a State Department-sponsored learning programme.

Yao Lifa, 50, a school official in the small city of Qianjiang, achieved nationwide celebrity as a fast-talking and tireless advocate of democratic votes, winning himself a place on the local Communist Party-controlled assembly.

But Yao Lifa's campaigning and mobilisation of disaffected farmers irked officials. Under a shadow of official disfavour, he failed to win re-election but threw himself into a life of organising other independent candidates.

He also signed the "Charter 08" petition issued last month demanding democratic political reform, which has pitted hundreds of dissidents and rights advocates against the Party. One of the organisers of the petition, Liu Xiaobo, is detained by police.

A spokeswoman in the U.S. Embassy in Beijing said Yao Lifa had been invited to a reception in Wuhan, along with other past participants in the visitor programme. She said the Embassy had nothing further comment.

Asked about Yao Lifa, a Chinese Foreign Ministry spokesman, Jiang Yu, said she did not know about the case.