A Chinese activist who investigated whether shoddy construction caused the deaths of thousands of children when their schools collapsed in a 2008 earthquake was released Thursday after finishing a five-year jail term, his lawyer said.

Writer and campaigner Tan Zuoren was heading home to Chengdu, the capital of the southwestern province of Sichuan, his lawyer Pu Zhiqiang told AFP by phone.

"He was released today. Now he's on the way home," Pu said.

Tan, 59, was sentenced for "inciting subversion of state power" in connection with several articles he published online about authorities' brutal crushing of the 1989 Tiananmen Square pro-democracy protests.

But he was arrested while investigating the deaths of thousands of children whose schools collapsed in a huge earthquake in Sichuan.

The 8.0-magnitude disaster in May 2008 left more than 80,000 people dead or missing.

But schools bore the brunt of the disaster, with 7,000 badly damaged and 5,335 pupils left dead or missing, according to authorities, fuelling angry accusations from parents that corruption had enabled low building standards.

Pu, a prominent Beijing-based rights lawyer who represented Tan at his trial in 2009, said the activist may continue to face surveillance and restrictions, especially during sensitive times.

This June will be the 25th anniversary of the Tiananmen crackdown, and authorities routinely restrict the movement of activists during such periods.

"The country owes him five years," Pu told AFP on Wednesday.

"First, this was a wrongful conviction. Second, given his circumstances his sentence was plenty long -- the maximum sentence was five years, and that's what the court gave him. Third, he served the entire sentence, not one day less."

When Tan was tried the high-profile Beijing-based dissident artist Ai Weiwei, who also investigated the school collapses, said he was detained and beaten by Chengdu police and blocked from testifying.

- 'He paid his price' -

Ai hailed the news of Tan's release Thursday but said: "He served every day, until the last day, and still now, he's under a kind of probation -- he doesn't have the right to speak up. So, I think that really is a shame."

Tan was a victim of "terrible judicial corruption", Ai said, noting that the court focused on his Tiananmen comments rather than the earthquake investigation.

"They know this obvious evidence points to the corruption and mishandling of the schools, which is the direct cause of the loss of the human lives of those young students," the artist told AFP.

An appeal in Tan's case was rejected in June 2010 after a hearing that "only lasted 12 minutes", Pu said at the time.

Rights groups denounced that ruling, with a top Asia official from Amnesty International calling it a politically-motivated outcome of "a grossly unfair legal process".

Searches for Tan's name on Sina Weibo, a Chinese microblogging site, were blocked Thursday, with users receiving a message that "according to relevant laws, regulations and policies, results for 'Tan Zuoren' are not displayed".

But Tan's supporters quickly spread the news of his release via Twitter, which is blocked by Chinese authorities but some people access through virtual private networks.

"He's such a fine man, very strong-willed and very determined, and he paid his price for the society he wants to help," Ai said. "He should know he has a lot of people supporting him."