A rights website based in the southwestern Chinese province of Sichuan was attacked by unknown hackers on Friday, the organizers said.
Attempts to load the Tianwang website redirected the user to a page with a multilingual message alleging that the site had engaged in "abuse" of the domain-name system, although normal service was restored later in the day.
"We started to receive various bits of feedback late [Thursday] night...that the Tianwang website was being automatically redirected," said the site's technician Pu Fei. "After the redirect, the wrong page was displayed."
"After investigating, we determined that it was caused by a premeditated network attack."
Pu said the Tianwang servers had been attacked, and basic changes made to the website.
"We basically resolved the problem later in the day, after 7:00 p.m. [local time], but we are still working on the security issues, and we will continue to do so," he said.
Pu said the group had already managed to pick up some traces of the hackers' activities, including their IP addresses.
"I can tell you very clearly that they included [IP addresses] inside China, and we can narrow it down quite clearly to the district and county [where they originated]," he said.
Tianwang, which was founded by prominent rights activist Huang Qi and began as a resource for relatives of those killed or injured in the military crackdown on the 1989 pro-democracy movement, gives detailed, online coverage of ordinary Chinese who seek to defend their rights in the face of official abuses of power.
Such stories rarely find expression in China's tightly controlled, state-run media, and are often deleted from social media sites soon after they appear.
Tianwang isn't the only rights website to have been subjected to hacker attacks in recent weeks.
The rights website Watchdog Net for Citizens and Public Opinion was also shut down in recent days, according to its founder, Li Xinde.
"This happened two or three days ago," Li said, adding that he suspected authorities unhappy with the website's work were behind the attack.
"The local governments don't dare to publicly oppose us in our rights and anti-corruption work, but behind the scenes, they are working hard to use their power and to ensure, via certain departments, that our site gets deleted or subjected to administrative measures."
"If all else fails, they will attack us with hackers."
Meanwhile, Tianwang's webmaster Huang said he was preparing for another possible jail term, as the ruling Chinese Communist Party gears up for a crucial leadership transition next month.
"For the past 14 years, the Tianwang rights website and service center has been continually subjected to oppression, and more of us have been to jail than any other rights group," Huang said. "Tianwang has also received more hacker attacks than any other rights website."
"Of course I feel as if I have let my family down," he said. "I have let my wife..., child, and mother down."
"My family have worked so hard for so many years [on my behalf]," he said.
Chengdu-based Huang was convicted of “illegally possessing state secrets” by the city's Wuhou District People's Court in 2009 after he called for an investigation into shoddy school construction blamed for the deaths of thousands of children during a massive 2008 earthquake in Sichuan province.
Before that, Huang had served five years in jail on "subversion" charges.
China has intensified a nationwide clampdown on dissident writers and rights activists ahead of the leadership change, set to take place at the 18th Party Congress in Beijing on Nov. 8.
Police have launched a "stability" drive, with many rights activists and dissidents reported to be under house arrest, either in their homes or in out-of-town locations like holiday resorts. Others are being held under criminal detention on charges they say are excuses to limit their freedom over the transition period.
China is listed as an "Enemy of the Internet" by Paris-based media freedom watchdog Reporters Without Borders, which says China is the "world's biggest prison" for netizens.
In its 2012 report on the internet, U.S.-based watchdog Freedom House said China had become a "key global source" for cyberattacks, including on domestic and overseas rights websites, and said the scale, organization, and targets of the attacks had led many experts to believe that they are linked to the government.
Reported by Qiao Long for RFA's Mandarin service. Translated and written in English by Luisetta Mudie.