More than 200 engineers, designers and managers at Google have demanded in an open letter that the company end development of a censored search engine for Chinese users, escalating earlier protests against the secretive project.

Google has described the search app, known as Project Dragonfly, as an experiment not close to launching.

But as details of it have leaked since August, current and former employees, human rights activists and US lawmakers have criticised Google for not taking a harder line against the Chinese government's policy that politically sensitive results be blocked.

Amnesty International launched a public petition yesterday calling on Google to cancel Dragonfly.

The human rights group said it would encourage Google workers to sign the petition by targeting them on LinkedIn and protesting outside Google offices.

Google has long sought to have a bigger presence in China, the world's largest internet market. It needs government approval to compete with the country's dominant home grown internet services.

About 1,400 of Google's tens of thousands of workers urged the company in August to improve oversight of ethically questionable ventures, including Dragonfly.

The nine employees who first signed their names on yesterday’s letter said they had seen little progress.

The letter expresses concern about the Chinese government tracking dissidents through search data and suppressing truth through content restrictions.

"We object to technologies that aid the powerful in oppressing the vulnerable, wherever they may be," the employees said in the letter published on the blogging service Medium.

The employees said they no longer believed Google was "a company willing to place its values over profits," and cited a string of "disappointments" this year, including acknowledgement of a big payout to an executive who had been accused of sexual harassment.

That incident sparked global protests at Google, which like other big technology companies has seen an uptick in employee activism during the last two years as their services become an integral part of civic infrastructure.