Google.com is currently blocked in China, according to watchdog group GreatFire.org.
"It's the first time since we started tracking online censorship in China in February last year that this has happened," the group said in a Friday blog post. "The subdomains www.google.com, mail.google.com, google-analytics.com, docs.google.com, drive.google.com, maps.google.com, play.google.com and perhaps many more are all currently DNS poisoned in China."
Looking at Google's online transparency report, which monitors the accessibility of its services around the world, traffic to Google products has dropped off in China in the last few hours.
"We've checked and there's nothing wrong on our end," a Google spokeswoman said when asked about the blockade.
According to GreatFire.org, anyone in China trying to access Google services are redirected to an IP address in Korea that doesn't serve any website at all.
"This means that none of these websites, including Google Search, currently work in China, unless you have a VPN or other circumvention tool," the blog said.
At this point, it does not appear that international versions of Google - like Google.co.uk - have been disrupted in China.
Google is the second most popular search engine in China behind Baidu and is among the top five most used websites in China on a daily basis, according to Alexa stats. GreatFire.org suggested, however, that the blockade could backfire on Chinese officials as more and more users realize that Google is blocked.
"Never before have so many people been affected by a decision to block a website," GreatFire.org said. "If Google stays blocked, many more people in China will become aware of the extent of censorship. How will they react? Will there be protests?"
Why now? China is currently holding its 18th Party Congress, which picks new leaders of the Communist Party of China. "The fact that Google is blocked now is surely no coincidence. The big question is whether it will be unblocked again once the congress is over. We will closely monitor developments," GreatFire.org said.
GreatFire.org said VPN and other tools are "working poorly or not at all." In the interim, Web users could try accessing Google sites directly using Google's IP addresses, which are not yet blocked. A full list is in the blog post.
Google has had a rather tumultuous relationship with Chinese officials in recent years. In January 2010, Google said there were attempts to hack into the Gmail accounts of Chinese human rights activists. At the time, Google pledged to no longer censor search results in China, even if that meant pulling out of the country entirely, and re-routed all Google.cn traffic to the uncensored Google.com.hk. Unsurprisingly, a Chinese minister warned of "consequences" if Google continued redirecting its results. Finally, the two parties settled on a hybrid solution so that Google could maintain its presence there.
In June, Google announced that it would begin displaying warnings to Search users in mainland China when their query was likely to result in an error beyond the search giant's control.