The authorities in China have made Google's services largely inaccessible in recent days, a move most likely related to the government's broad efforts to stifle discussion of the 25th anniversary of the crackdown on pro-democracy demonstrators in Tiananmen Square on June 3 and 4, 1989.
In addition to Google's search engines being blocked, the company's products, including Gmail, Calendar and Translate, have been affected.
This is not the first time China has taken aim at Google and its users there. The authorities in China blocked Google for 12 hours in 2012, according to GreatFire.org, an independent censorship-monitoring website, which published a blog post about the recent problems on Monday. But the recent crackdown is more severe, and there was no indication of how long it would last.
"This is by far the biggest attack on Google that's ever taken place in China," said a co-founder of GreatFire.org, who asked to remain anonymous to prevent retaliation by the authorities. "Probably the only thing comparable is when the Chinese government first started blocking websites in the 1990s."
While Internet users in mainland China could reach international versions of Google search until a few days ago, "all Google services in all countries, encrypted or not, are now blocked in China," GreatFire.org said in the blog post. These include the Chinese-language version based in Hong Kong, Google.hk, and Google.com, Google Australia and others.
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Other services with no direct search function, including the company's Picasa photo program, Maps service and Calendar application, were also inaccessible to most users on Monday. "It is the strictest censorship ever deployed," the blog said.
Unlike the websites of Facebook, YouTube, Twitter and The New York Times, which are reliably blocked by the authorities, Google services are being disrupted in a way that affects about 9 out of 10 Chinese users, according to GreatFire.org. By allowing some access, "the Chinese government is trying to pin the blame on Google," the GreatFire co-founder said.
Google says that it is not the problem.
"We've checked extensively, and there are no technical problems on our side," said a Google spokeswoman, who declined to elaborate.
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Google's traffic from China on all its services fell 50 percent over the weekend, according to the company's transparency report.
As most Internet users in China can attest, Google's services have been subject to varying degrees of interference since 2010, when the company shut down its Internet search service in China amid accusations of government censorship and intrusions by state-backed hackers. The move prompted angry denunciations by the Chinese government, but many young people responded by placing mourning wreaths at Google's headquarters in Beijing, a testament to the company's popularity in the country.
Since then, Google has been directing users to an uncensored search engine in Hong Kong.
The government has in the past denied that it interferes with Google services in China, but many users complain that gaining access to Gmail is difficult and at times nearly impossible.
The co-founder of GreatFire.org said complaints began appearing on Chinese social media last week but were quickly erased by censors.
"Sigh ... my Google calendar is dead again," a user on the popular Sina Weibo microblog wrote on Monday. "Every year it's sensitive, this year even more so." The post was swiftly deleted.
Whether the blockage is permanent or just a temporary measure that will ease after June 4 is unclear. Government offices in China were closed Monday for a national holiday.
In recent weeks, the authorities have waged a particularly aggressive campaign against those who might seek to discuss or commemorate the events of 1989, detaining dozens of dissidents, scholars and legal defenders. Some of those detained are facing criminal charges, a development that rights advocates say goes beyond previous efforts to stifle public commemoration of the crackdown. In an effort to foil online discussion, code words for the crackdown, including "6-4-89" and "May 35," have also been blocked.
"They're locking up everyone that they can and blocking everything they can," said Jeremy Goldkorn, director of Danwei, a website that tracks the Chinese news media and Internet.
But even as the Chinese government continues to fortify its complex online censorship regimen, commonly known as the Great Firewall, software developers who support the free flow of information have been creating more innovative products that allow users to break through. Millions of people in China rely on proxy servers, virtual private networks and other methods to skirt Internet controls, although those measures are often subject to interference.
In March, Google began encrypting what is known as search by default in China. The secure system permits users to conduct uncensored searches, an act the government could prevent only by blocking direct access to Google. The authorities have now locked that electronic door, unless Internet users have the right software to pry it open.
At least one workaround is protected in a place censors may find hard to reach: the cloud. The activists at GreatFire.org say they have developed an "unblockable" Google mirror website that relies on encrypted cloud computing. To take it down, the government would have to block online systems used by many companies in China — a move that probably would have significant economic repercussions.