Mr Hu's family say he is not receiving medical care in prison
Hu Jia is one of China's most prominent activists.
He has a reputation for being unafraid to take on the government over a range of issues, including religious and political freedom, HIV/Aids and the environment.
Mr Hu became particularly outspoken in the run-up to the 2008 Olympic Games in Beijing, publishing an open letter to the Chinese government titled The Real China and the Olympics, which called for an end to human rights abuses in the country.
In 2007, Mr Hu was placed under house arrest after he and his wife, Zeng Jinyan, made a documentary about their experiences of living under constant police surveillance.
Prisoners of Freedom City showed the couple remained under police guard even when Ms Zeng went to hospital to give birth in November 2007.
Mr Hu had to get permission from the police to take her to the hospital.
In February 2008, Mr Hu was formally arrested and charged with "inciting subversion of state power and the socialist system", a charge often cited to jail opponents of the state.
The charges related to five articles Mr Jia had written as well as interviews he gave to journalists in which he was critical of the authorities.
The state-run Xinhua news agency reported that he had "spread malicious rumours, libel and instigation".
Mr Hu was sentenced to three-and-a-half years in jail and deprived of political rights for one year.
The verdict attracted condemnation from the West, with the US expressing "dismay" while the European Union called for Mr Hu's immediate release.
Human rights groups say Mr Hu's arrest was part of Chinese attempts to crack down on vocal opponents of the state before the Olympics.
Mr Hu suffers from liver disease due to Hepatitis B infection. Amnesty International says the family has been unable to provide him with medicine.
He is receiving some medication from prison authorities, but his family are concerned that this may not be adequate.
Human rights group say they are also concerned for Ms Zeng, who remains under effective but unofficial house arrest, along with her young daughter.
CCTV is reported to be installed in the apartment and her movements outside her home are restricted by the police, says US-based Human Rights Watch.
Ms Zeng, who has become a regular blogger since her husband's arrest, was reported to have been removed from Beijing by the police during the Olympics.