Hu Jia was reunited with his family early Sunday after serving three and a half years in prison, his wife said. Above, Hu and his wife in 2007. (FREDERIC J. BROWN/AFP/GETTY IMAGES)

– Hu Jia, a prominent Chinese dissident whose activism on behalf of the environment and AIDS suffers landed him in prison for the last three and a half years, was released in the pre-dawn hours Sunday and returned to his home in Beijing, his wife said in a Twitter posting.

“Sleepless night,” Hu Jia’s wife, Zeng Jinyan, posted on Twitter. “Hu Jia arrived home at 2:30 in the morning. Safe, very happy. Needs to rest for awhile. Thanks, everyone.”

Hu was arrested in December 2007, and convicted by a Beijing court in April 2008 of “incitement to subvert state power,” a catchall law frequently used to target critics of China’s authoritarian Communist government. Hu, 37, began as an environmental campaigner, and took up the cause of victims of HIV and AIDS in China, before his activism led him to become increasingly critical of the government.

At the time, human rights groups and others said the authorities wanted to remove an outspoken critic before the start of the 2008 Beijing Summer Olympics, which cast an international spotlight on the Communist government’s human rights record.

Hu is the second high-profile activist to be released in the last four days. On Wednesday, renown dissident artist Ai Weiwei was allowed to return to his home in Beijing after 80 days held incommunicado, on what the government said was tax evasion charges. Ai’s family and lawyers called the accusations a pretext, and said that Ai was targeted for his outspoken anti-government views.

But Hu, like Ai, faces tough restrictions after his release, including being prohibited from speaking to the media and being deprived of his “political rights” for one year, meaning he cannot vote in local elections which are dominated by the Communist Party.

Another activist, Chen Guangcheng – a blind lawyer who exposed forced abortions and forced sterilizations under the government’s family planning program — was released last year after four years in prison, but has been kept under an unofficial kind of house arrest in his village in Shandong Province. He is unable to speak to the media, and journalists who tried to visit him were chased away by local thugs.

On Sunday, a cell phone for Hu’s wife was powered off, and his mother did not answer the telephone Sunday. Some human rights groups and lawyers were concerned that the government was using strict post-jail conditions to continue to silence unwelcome voices of dissent.

“Hu Jia should never have been imprisoned in the first place,” said Sophie Richardson, the Asia advocacy director for the New York-based group Human Rights Watch, in a statement. “If that injustice is compounded by another form of detention, it will show just how shallow the Chinese government’s ‘rule of law’ commitments are.”

“As a friend, I hope he can keep a low profile,” said Hu’s lawyer, Li Jinsong. “He should avoid unnecessary trouble. It’s time for him to take care of his family.” Li added, “We worry that the experience of Chen Guangcheng will be repeated with Hu Jia.”

Since late last year, the Chinese government has engaged in a sweeping crackdown of activists, first because of its outrage over the Nobel Peace Prize awarded to jailed dissident writer Liu Xiaobo, whose wife remains under house arrest and cut off from the outside world. Earlier this year, the regime was jarred by popular protests against authoritarian regimes spreading across the Middle East and North Africa, which prompted anonymous online calls for similar “jasmine revolution” protests in China.

Hu suffers from cirrhosis of the liver and other ailments, but was repeatedly denied a medical parole during his confinement. In 2008, the European Parliament awarded Hu its highest human rights honor, the Sakharaov Prize. At the ceremony in Stasbourg, France, Hu was represented by an empty chair.

Washington Post researcher Zhang Jie contributed to this report.