The Internet giants are currently working on a project entitled 'Dragonfly', which would be a censored version of Google's search engine developed with the help of the Chinese government, and would block terms including "human rights" and "religion".
But in a letter published online, 60 Google employees stated the project would only help state surveillance, and called for the project to be cancelled.
Their plea has been backed by charity Amnesty International, which said it was at odds with the company's values.
According to the BBC, Dragonfly would "enable censorship" and help the Chinese government's disinformation campaigns.
In the letter, the employees wrote: "Dragonfly in China would establish a dangerous precedent at a volatile political moment, one that would make it harder for Google to deny other countries similar concessions.
"We object to technologies that aid the powerful in oppressing the vulnerable, wherever they may be."
Anna Bacciarelli, Amnesty International's researcher on technology and human rights, said Google staff don't want to be part of plans to bring further censorship to China, and noted that in August, more than 1,400 of the company's employees called on it to do a better job of policing ethically troubling development jobs.
Google declined to comment on the letter, and said its work with China on a search engine was "exploratory".