The Chinese Government has blocked several Google services and other websites ahead of this week's 25th anniversary of the Tiananmen Square protests.
The Beijing Government, which already blocks Facebook, Twitter and Google-owned YouTube, has stopped servers accessing Google's main search engine and Gmail, according GreatFireWall.org, a nonprofit group that monitors censorship in the country.
"It is not clear that the block is a temporary measure around the anniversary or a permanent block," the group said.
"But because the block has lasted for 4 days, it's more likely that Google will be severely disrupted and barely usable from now on."
It added the last time it monitored such a disruption was in 2012, when it only lasted 12 hours.
Google has yet to make an official statement but a spokesperson told Reuters: "We've checked extensively and there's nothing wrong on our end."
In 2010, Google moved its Chinese search engine service out of China to Hong Kong.