The Prey Sar prison authorities denied Mr. Rong Chhun's family access to visit him and bring him medical supplies, foods and other necessities.
Mr. Rong Chhun is Cambodian Independent Teachers Association (CITA) director.
The civil association officials say that Mr. Rong Chhun's family has written an official and legal letter to the Ministry of Interior but did not receive a written permission, quoting the authorities as saying that the situation is not very good.
The CITA's administrative director Leng Bun Hong says that Mr. Rong might not be able to see at all if untreated.
Other activists' families were allowed to visit the detainees.
Mr. Rong's lawyer Tae Chamnan says that he plans to see his client this week and will ask to have a doctor go in to see Mr. Rong.