A campaign organisation that avoids Chinese website blocks has said it has come under distributed denial of service attack.
Greatfire called the attack, which aimed to take sites offline, an attempt to enforce censorship and added that it did not know who was behind it, but pointed out that it coincided with pressure from Chinese authorities.
One expert has called the attack "censorship by brute force" saying it could put Greatfire out of business.
Greatfire has tracked which sites are blocked in China and recently began offering a mirroring service to try to restore them for Chinese users.
Similar to a campaign started by Reporters Without Borders last week, it set up content distribution networks using the same hosting services as many entities on which China relies.
Those networks were said to have created copies of banned websites, making them available to Chinese users.
The method, in theory, provided protection to Greatfire as, by making sure that the blocked websites remained inaccessible attackers, they would have had to take down the whole hosting service, including many sites that China wanted to remain live.
Professor Alan Woodward said in practice, however, the attackers managed to find the individual URLs of the sites the authorities sought to block and bombarded them in a more targeted attack.
He said: "It is difficult not to conclude that someone, most likely a government annoyed that Greatfire has enabled previously censored websites to neuter their censoring technology, has decided to fight back.
"The big question will be whether the big companies that run the CDNs... will actually key the charges Greatfire is being forced to incur, or whether they believe the censorship avoiding method should be supported when under attack in this way."
In a statement, Greatfire said the attacks started on the 17th March and were receiving up to "2 billion requests per hour, which is about 2,500 times more than normal levels."
The statement said: "We are under attack and we need help.
"Likely in response to a recent story in the Wall Street Journal (WSJ), we've experienced our first ever distributed denial of service (DDoS) attack.
"This tactic is used to bring down web pages by flooding them with lots of requests - at the time of writing they number 2.6 billion requests per hour. Websites are not equipped to handle that kind of volume so they usually 'break' and go offline."
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