Two of the men had something else in common. Mr. Huang and Mr. Liu headed rights organizations that have come under scrutiny, and their detentions may be related to the pending implementation of a law on nongovernmental organizations that puts new restrictions on foreign groups operating in China. Such groups will be required to register with the Ministry of Public Security, and the police will have the right to scrutinize their operations, including financing. They must also find a Chinese partner. The law also makes Chinese groups that receive funding from outside the country more vulnerable.

Image Huang Qi, in a family photograph.

The disappearances of Mr. Liu, who leads Civil Rights and Livelihood Watch, and Mr. Huang, who runs the 64 Tianwang Human Rights Center, may be a prelude to the law’s implementation.

“This may show the mind-set of the authorities as they come close to implementing the NGO law,” William Nee, a researcher for Amnesty International who is based in Hong Kong, said by phone.

The mobile phones of Mr. Jiang, Mr. Liu and Mr. Huang were either turned off or appeared to not be working. A police officer in Chengdu, the capital of Sichuan Province, said that Mr. Huang had been arrested in Chengdu by officers from the nearby city of Mianyang. The police in Suizhou, Mr. Liu’s hometown, said the city government was handling Mr. Liu’s case but denied any further knowledge. The police in railway stations in Changsha and Beijing, as well as in Mr. Jiang’s hometown in central China, had no information on his whereabouts.

Mr. Chen, Mr. Jiang’s lawyer, said his client had been moving from place to place in and around Beijing for three years, trying to avoid arrest. He had been detained for months in 2011, amid an earlier crackdown that came in the wake of the movements that swept authoritarian leaders in Tunisia and Egypt from power. China feared the so-called Jasmine Revolution would come to China as well.

Mr. Jiang was detained that year for two months, telling rights groups of his abuse at the hands of his interrogators, according to an account of his life on the website of China Change, a group based in the United States.

“If, as is strongly likely, this was an act carried out by state agents, then this would be an enforced disappearance, which is a crime under international law,” Mr. Nee of Amnesty International said. “Jiang Tianyong seems to be placed outside the protection of the law, which makes him very vulnerable to torture and other human rights violations.”