Xu Zhiyong, whose "New Citizens' Movement" advocated working within the system to press for change, was detained in 2013 and subsequently convicted of "gathering a crowd to disturb public order".
One of the group's main demands had been for officials to publicly disclose their assets, a demand taken against the backdrop of the ruling Communist Party's own efforts to crackdown on deep-seat corruption under President Xi Jinping.
Xu's lawyer, Zhang Qingfang, told Reuters he had brought Xu up to speed with "events on the outside", including the death of fellow activist and Nobel Peace Prize laureate, Liu Xiaobo. He said Xu was "upset" upon hearing the news.
Zhang said Xu, who was released from his jail on Beijing's outskirts on Saturday morning, was in good physical condition and had few immediate plans beyond spending time with family.
At the height of Xu's activism, he attracted hundreds of supporters who participated in activities related to the movement, having first gained prominence in 2003 for helping victims of tainted baby formula and migrant workers without access to healthcare and education.
It prompted a crackdown from the Communist Party, which swiftly crushes any perceived challenge to its rule.
"The idea of the New Citizens Movement is not to overthrow, but to establish," he wrote in a 2010 essay. "It's not one social class displacing another social class, but allowing righteousness to take its place in China."
As international rights groups and foreign governments call for Chinese authorities to guarantee freedom for Liu Xiaobo's widow, Liu Xia, Xu's supporters have also expressed concern whether he will remain under close watch or effective house arrest. Some said on social media they were barred by security guards and plain-clothed officers from entering Xu's apartment compound on Saturday.
Other high-profile and politically sensitive prisoners released from jail, including rights lawyer Pu Zhiqiang and journalist Gao Yu, remain closely watched by Chinese authorities.
"I hope he will be completely free," Zhang said.
Xu taught law at a Beijing university and ran in a local election. He became prominent over a drive to abolish "custody and repatriation" powers, a form of arbitrary detention used by local governments to sweep homeless people off the streets.
(Editing by Ben Blanchard and Jacqueline Wong)
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