An online support website, Change.org, with a petition to free Chinese artist and activist Ai Weiwei was attacked by hackers who are believed to be from China.
The founder of the website based in the US, Ben Rattray, said the online petition platform suffered denial of service (DDoS) attacks on 17 April after hackers targeted it for running a campaign demanding the release of WeiWei.
Rattray said, "We do not know the reason or exact source of these attacks."
"All we know is that after the unprecedented success of a campaign by leading global art museums using our platform to call on the Chinese government to release Ai Weiwei, we became the victims of highly sophisticated denial of service attacks from locations in China."
Rattray added that staff have stabilised the platform now.
WeiWei is best known for his contributon in the construction of the Bejing National Stadium for the 2008 Olympics and his Sunflower Seeds exhibit at the Tate Modern in 2010. He has also fought against government corruption over the past few years.
The Chinese police took the 53-year-old artist into custody at an airport in Bejing earlier this month, after which his whereabouts is unknown. Several friends and colleagues of WeiWei are also missing.
The online petition to free WeiWei, which was launched two weeks ago by leading members of the international arts community, was signed by around 90,000.
Rattray said that they would continue to fight for the release of Weiwei.
Rattray said, "Rather than take the appeal down, we will continue to stand with Ai Weiwei's supporters to defend freedom of speech."