A Chinese dissident who founded one of the country's pioneering human rights websites has been arrested for possessing state secrets.

Huang Qi, who set up 64Tianwang – the numbers refer to the date of the Tiananmen Square massacre, on June 4 1989 – was detained in the south-western city of Chengdu, capital of Sichuan province, his mother said.

"They didn't say when he would be freed - first they have to do an investigation," Pu Wenqing, 74, said.

Pu said she was informed by the police of the arrest yesterday and had been unable to speak to her son since then. The reason for Huang's arrest, possession of state secrets, is an ill-defined offence often used to clamp down on dissent.

Officials in Chengdu declined to comment on his arrest.

Huang's website, founded a decade ago, allowed people, for a fee, to post information about missing friends and family members.

The watchdog Human Rights in China said Huang had been detained on June 10 after visiting areas affected by the earthquake in May, centred in Sichuan province.

The group said Huang's purpose there had been to write about parents whose children had died or were missing in the quake.

"This is another illustration of how a person who is only trying to help might find himself snared by China's state secrets laws," the watchdog's executive director, Sharon Hom, said.

"This use of the law as a sword hanging over rights activists, such as Huang Qi, contradicts the reported 'new media openness' in China following the Sichuan earthquake."

The Paris-based rights monitoring group Reporters Without Borders said last week that three men, likely agents from China's ministry of state security, had forced Huang into a car.

It said his arrest might be linked to articles he had posted criticising the government's response to the magnitude-7.9 quake that killed almost 70,000 people in Sichuan.

China's security forces have begun to clamp down on dissent after initially tolerating independent reporting on the quake and allowing public complaints by parents who blame corruption and shoddy construction for school collapses that killed their children.

Huang has long been one of China's most outspoken activists. Earlier this decade, he served a five-year prison sentence on subversion charges linked to politically sensitive articles posted on his website.

Since his release, in 2005, Huang, who is in his mid-40s, has supported causes ranging from aiding families of people killed in the 1989 military crackdown on pro-democracy protests in Beijing to publicising the complaints of farmers involved in land disputes with authorities.