Googles headquarters in Beijing. Surfers in China might have found it harder to access an uncensored Google via Greatfire.org since the attack. Photograph: Sinopix/REX

An advocacy group that helps internet users inside China bypass blocks on censored content says it is suffering a denial-of-service attack disrupting its operations.

US-subsidised Greatfire.org says the attack started two days ago and traffic is 2,500 times above normal. It has affected mirror, or duplicate, websites that it has set up via encrypted web services offered by companies such as Amazon.

Greatfire.org said the attack has interfered with visitors to sites including Boxun.com, which publicises allegations of corruption and human rights abuses inside China, German provider Deutsche Welle, and Google.

The statement from a co-founder of the group, who goes by the pseudonym Charlie Smith, said its not clear who is behind the attack, but it coincides with increased pressure on the organization over the last few months and public criticism from Chinese authorities.

The Chinese government blocks thousands of websites to prevent what it deems politically sensitive information from reaching Chinese users, an effort dubbed the Great Firewall.

According to the free-expression watchdog Freedom House, since late 2013 Greatfire.org has been hosting content on domains owned by Amazon and other major companies, which officials cannot risk censoring because of their large commercial footprint within China.

Smith said the current denial-of-service attack that is flooding the mirror websites is costing the group up to $30,000 per day in bandwidth.

Greatfire.org says it gets its funding from a variety of sources, including from people and organizations inside China. The Open Technology Fund, a US-government-backed initiative to support internet freedom, says on its website it provided Greatfire.org with $114,000 in 2014.

Zhu Haiquan, spokesman of the Chinese Embassy in Washington, said: As we have always stated, Chinese laws prohibit cybercrimes of all forms. The Chinese government is making great efforts to combat cybercrimes and safeguard cybersecurity. Jumping to conclusions and making unfounded accusations is not responsible and is counterproductive.

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Chinese anti-censorship group Greatfire.org suffers massive hack