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For years, a group of anonymous activists known as GreatFire.org has monitored online censorship in China, provided access to blocked websites and collected messages deleted by censors.

This week, unidentified hackers have tried to put an end to those activists’ efforts with an unprecedented attack. In a post to its blog Thursday, GreatFire.org said it has experienced a massive so-called denial of service attack.

The method is one that hackers frequently use to foil websites by flooding them with multiple requests — so many that they go offline and viewers see a blank page. GreatFire.org creates encrypted versions of 12 websites that are blocked in China. These are known as mirrored websites and grant users within China access to the content.

On Thursday, GreatFire.org said it was receiving 2.6 billion requests an hour for its mirrored websites. On Friday, access to the mirrored websites was inconsistent in China.

“We are under attack and we need help,” GreatFire.org said. “This kind of attack is aggressive and is an exhibition of censorship by brute force. Attackers resort to tactics like this when they are left with no other options.”

Meanwhile, the Reuters news agency reported that its website was inaccessible in China on Friday. The websites of The New York Times, The Wall Street Journal and Bloomberg have previously been blocked in China.

“Reuters is committed to practicing fair and accurate journalism worldwide,” said Caroline Drees, a spokeswoman for Reuters.

GreatFire.org’s name is inspired by the Great Firewall, the term often used to describe China’s Internet censorship. About two million people in China access GreatFire’s websites each month, a co-founder of the group who uses the pseudonym Charlie Smith, wrote in an email exchange.

It was unclear who was responsible for the attack, which began Tuesday from inside and outside China, Mr. Smith wrote. GreatFire.org noted in its blog post that its tactics were the recent subject of a report in The Wall Street Journal, which appeared online Monday.

The timing for the attack was a mystery. “Maybe that WSJ story,” Mr. Smith wrote. “Maybe because there have been some excellent Chinese-language news pieces and perhaps somebody who supports the authorities took issue with them. In the past there has rarely been rhyme or reason on the timing of such attacks.”

GreatFire.org’s mirroring services provide unrestricted access within China to a range of websites, including itself and the Chinese language version of The New York Times, which has been regularly blocked in China. Some of the others are Deutsche Welle, BBC News, China Digital Times, Google.com, and Boxun, a Chinese-language news website. GreatFire.org says it does not mirror The Wall Street Journal. GreatFire.org works directly with some, but not all, of the websites it mirrors.

GreatFire.org is partly funded by Open Technology Fund, a United States government-financed initiative under Radio Free Asia. Last year it provided $114,000 in funding, according to its website. Mr. Smith declined to comment on any financial backing.

The Chinese government has in the last year ramped up efforts to prevent its citizens from accessing critical news coverage from abroad and from communicating on social media platforms that the government cannot directly censor.

China has long disrupted many of Google’s services. Facebook, Twitter and YouTube remain blocked. LinkedIn agreed to censor its content to operate in the Chinese market last year.

GreatFire’s mirroring websites circumvent the Great Firewall by channeling Internet traffic through cloud services, such as one available from Amazon. The difficulty for the Chinese government is that it can’t just shut off Amazon’s service, because it is used broadly by many major Chinese corporations.

Emails to the Chinese Foreign Ministry and the Chinese embassy in Washington went unanswered as of Friday evening. The Associated Press quoted Zhu Haiquan, a spokesman for the embassy, as saying: ‘‘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.’’

The hacking is costing GreatFire up to $30,000 per day in additional charges, Mr. Smith wrote. However, he wrote that he was confident that the group’s services would not be disrupted for good.

“The authorities will not cut off access to the world’s Internet infrastructure because they know that it is valuable,” Mr. Smith wrote. “If anything, this should accelerate the development and deployment of collateral freedom as a strategy to achieve freedom of access to information in countries like China.”