Google must abandon its development of a censored search engine for China, dozens of NGOs demanded on Tuesday, warning personal data would not be safe from Beijing authorities.
A global coalition of 60 human rights and media groups wrote to Google chief executive Sundar Pichai urging him to scrap the "Dragonfly" project, which has already sparked opposition from the US tech giant's own staff.
Pichai in October acknowledged publicly for the first time that the company is considering a Chinese search engine, saying it could offer "better information" than rival services.
But Reporters Without Borders (RSF), a signatory to the letter, said Pichai must think again.
"In addition to being totally opaque and contrary to the values that Google relies on, the Dragonfly project offers no guarantee of data confidentiality," said Cedric Alviani, director of RSF's East Asia Office.
"Beijing collects massive quantities of personal data for purposes of censorship and surveillance, including against journalists and their sources."
RSF said China ranked 176 out of 180 countries in its Freedom of the Press Index.
Google shut down its search engine in China in 2010, refusing Beijing's requirement to censor search results.
Pichai has described Dragonfly as an effort to learn what Google could offer if it resumed its search operations in the world's second largest economy.
However, opposition to the plans is growing.
Amnesty International warned last month that a search application designed to filter out censored content from results could damage all internet users' trust in Google, the world's leading search engine.
Some 90 Google employees in November posted an open letter saying the service would set a dangerous precedent.
US internet titans have long struggled with doing business in China, home of a "Great Firewall" that blocks politically sensitive content, such as the 1989 Tiananmen massacre.
Twitter, Facebook, YouTube and The New York Times website are blocked in China, but Microsoft's Bing search engine continues to operate.