Chinese police have detained a prominent human rights activist and Aids campaigner who has fought several high-profile cases.
Mr Hu had been under police watch since July
More than 20 plain clothed policemen took Hu Jia from his home in Beijing, according to his wife, Zeng Jinyan.
Mr Hu, 33, was said to be collecting data on the detention of activists and has been under house arrest since July.
Ms Zeng said she believed the arrest was part of a wider crackdown on human rights campaigners.
Last month, Chen Guangcheng, who had campaigned against forced sterilisation and abortion, was jailed for public order offences. His lawyers were detained ahead of his trial.
Gao Zhisheng, a well-known lawyer who represented members of the banned Falun Gong group and villagers who accused local officials of stealing their land, was also detained last month.
"In the past month, I keep hearing about people missing, arrested, I believe it's part of a campaign," Ms Zeng said.
She said the men banged on the door at 0830 (0030 GMT) and told Mr Hu to go with them to a local police station.
"We can only wait and see if this will be just an interrogation or something longer," she said.
The couple had been under constant police surveillance by police since July, Ms Zeng said.
Mr Hu, 33, had worked to protect the rights of Aids sufferers and had also protested against the detention of other activists.
He had reportedly been trying to arrange a lawyer for Gao Zhisheng.
On the night before his arrest, he received a telephone call from Mr Gao's wife, Geng He, after which telephone and internet access were cut, his wife said.