GreatFire.org said in a blog post that the government appeared to have begun targeting Google Inc's main search engine and Gmail, among many other services, since at least last week, making them inaccessible to many users in China.
It added that the last time it monitored such a block was in 2012, when it only lasted 12 hours.
"It is not clear that the block is a temporary measure around the anniversary or a permanent block. But because the block has lasted for four days, it's more likely that Google will be severely disrupted and barely usable from now on," the advocacy group said.
Asked about the disruptions, a Google spokesman said: "We've checked extensively and there's nothing wrong on our end."
Google's own transparency report, which shows details about its global traffic, showed lower levels of activity from China starting from about Friday, which could indicate a significant amount of disruption.
Reuters was unable to reach any government officials for comment as Monday is a national holiday in China. Beijing typically responds to such reports by saying that all Internet companies operating in China have to obey the law.
Google in 2010 moved its Chinese search engine service out of China, the world's second-largest economy, citing rampant censorship, and now operates it from Hong Kong.
For the ruling Communist Party, the 1989 demonstrations that clogged Tiananmen Square in Beijing and spread to other cities remain taboo, particularly on their 25th anniversary.
The government has detained several activists last month after attending a meeting about the protests, including prominent rights lawyer Pu Zhiqiang, prompting concern in the United States and Europe.
The anniversary of the date on which troops shot their way into central Beijing in 1989 has never been publicly marked in mainland China, though every year there are commemorations in Hong Kong.
The government has never released a death toll for the crackdown, but estimates from human rights groups and witnesses range from several hundred to several thousand.
China already has strict controls on what can be said online, and the government has been further tightening those restrictions.
Users of China's popular Twitter-like service Weibo sounded off about the Google blockage.
"Those officials are driving me crazy with this!" wrote one user.
© Thomson Reuters 2014