The social action website Change.org has said it is under continuing attack from hackers after it hosted a petition calling for the release of detained artist Ai Weiwei that garnered more than 90,000 signatures.
Attackers traced to China had begun distributed denial of service (DDoS) attacks on Monday, which temporarily brought down the site, the site's editor said.
"The attacks have not subsided, but our engineering team has done an incredible job mitigating their effects and it seems, for now at least, that they have managed to find a way around this," Change.org editor Benjamin Joffe-Walt said by e-mail on Wednesday.
He said the action was apparently linked to the success of a petition run by art museums on the site to call for Ai's release.
"The initial attack originated from within several China-allocated IP blocks and China was the origin of the original probing and DDoS," Joffe-Walt said.
"Since then the attacks have expanded in location and intensity."
"While we cannot confirm the motivation or exact origin of the attacks within China, it seems pretty clear it's in response to the viral success of the Ai Weiwei campaign being led by leading global art museums on our platform," he said.
Joffe-Walt said the site organizers had asked for help from the U.S. State Department and FBI, but had received no reply. The Chinese government had made no response to attempts by Change.org employees to contact them either.
According to Change.org statistics, more than 90,000 people in 175 countries signed the petition calling for the release of Ai, who was detained earlier this month pending investigation for "economic crimes."
While Ai's wife has since been questioned by the tax authorities, it was unclear whether this related to his detention. No formal notification has been given of Ai's detention, any charges against him, or his whereabouts, his relatives say.
The Change.org petition was started by New York's Guggenheim Museum and backed by the Museum of Modern Art, New York, and the Tate Modern, London, as well as the Association of Art Museum Directors.
By Wednesday it had garnered 93,338 signatures, including many from inside China.
Addressed to cultural minister Cai Wu, the petition expressed "disappointment" at Ai's detention.
"We members of the international arts community express our concern for Ai’s freedom and disappointment in China’s reluctance to live up to its promise to nurture creativity and independent thought, the keys to 'soft power' and cultural influence," the petition said.
"Our institutions have some of the largest online museum communities in the world," it said. "By using Ai Weiwei’s favored medium of 'social sculpture,' we hope to hasten the release of our visionary friend."
Change.org said the hacker attacks began early Monday, making the site "completely inaccessible" for a few hours, and prompting the group to ask the FBI and State Department for help.
"We do not know the reason or exact source of these attacks,” founder Ben Rattray said in a statement on the group's blog.
"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."
Change.org has been blocked in China at various points over the last few years, the blog post said.
Beijing has brushed off recent calls for Ai's release from Britain, France, Germany, and the United States, whose outgoing ambassador Jon Huntsman mentioned Ai's detention in a recent speech.
Chinese authorities sometimes try to silence critics by accusing them of tax violations or other nonpolitical crimes.
Ai is the most famous target so far in a recent crackdown on dissidents apparently sparked by anonymous online calls for a "Jasmine revolution" inspired by recent uprisings in the Middle East.
An official editorial said last week that his actions were "ambiguous" in law, and close to a "red line."
Ai, 57, is a top artist who helped design Beijing's Bird's Nest Stadium for the 2008 Olympics and is currently exhibiting his "Sunflower Seeds" installation at London's Tate Modern gallery.
An inveterate Twitter user, Ai has taken part in a number of campaigns to protect the most vulnerable in Chinese society, including an online memorial installation which recorded the names of thousands of children killed in the collapse of school buildings during the devastating 2008 Sichuan earthquake.
Reported by Luisetta Mudie.