A Chinese activist who investigated the deaths of children in schools that collapsed in the Sichuan earthquake was today jailed for five years for subversion, his lawyer said.

The court in Chengdu sentenced Tan Zuoren over comments he made in online articles about the violent crackdown on Tiananmen Square protests in 1989. But he and his supporters believe he was detained owing to his research into the deaths of thousands of pupils. Charges related to his investigation were ignored in the verdict.

"The court was very smart. They took out any mention of the earthquake from the verdict because they are afraid of referring to it," said his lawyer, Pu Zhiqiang.

The quake in the south-western province in May 2008 left almost 90,000 people dead or missing. But parents demanded to know why many schools collapsed even when buildings around them stood firm. As public outrage about poor quality construction grew, authorities stamped out any discussion of the matter, harassing parents who protested.

Tan, 55, received the maximum sentence for subverting state power, highlighting what human rights groups describe as an increasingly punitive environment for dissidents. He plans to appeal.

The lawyer of another man who campaigned over the deaths of Sichuan children said today that a court had upheld his three year sentence on appeal. Huang Qi, a well known activist, was convicted in November of revealing state secrets after trying to gather information on shoddy school construction.

Tan's wife, Wang Qinghua, was not allowed to attend the 10-minute sentencing session. "Even one day of imprisonment is too much. He only exercised his freedom of expression and addressed corruption from his own conscience," she told Amnesty International.

"Tan's case is the most important one to take place recently, because it is a sign of a huge step backwards in China's judicial ethics and independence after decades of reform and opening," said Ai Weiwei, a leading artist who has also attempted to tally the number of pupils who died in the disaster.

"Tan Zuoren received such a serious punishment only for believing or writing in his [online] diaries that there were problems with the earthquake. It is ridiculous. Though China claims to the world that it is a major country, the case just shows how fragile and lacking in confidence it is."

Hong Kong radio station RTHK said police tried to block nine journalists from the territory from interviewing Tan's lawyer outside the court.

Chen Yunfei, another Sichuan activist, tweeted that he had wanted to attend the sentencing but that police arrived at his house last night to stop him doing so.

"[Tan's] arrest, unfair trial and now the guilty verdict are further disturbing examples of how the Chinese authorities use vague and over broad laws to silence and punish dissenting voices," said Roseann Rife, Asia-Pacific deputy director at Amnesty.

"The Chinese authorities cannot continue to claim that they are dealing with human rights defenders according to the law when they violate so many of their own legal procedures in cases like this."

Tan stood trial in August last year, but his lawyers were unable to call witnesses to testify or to show video footage they had prepared. Police also detained and threatened supporters including Ai.