More than 1,000 Google employees on Friday demanded that the tech giant explain why it had dismissed a Black artificial intelligence ethics researcher.

Timnit Gebru tweeted early in the week that Google told her a resignation she had not proffered was accepted, after she sent a message to an internal group lamenting the "silencing of marginalized voices."

She had also told the "Brain Women and Allies" group in the email that she had been ordered to retract a research paper.

Gebru is an outspoken diversity advocate and co-founder of a group devoted to increasing Black talent in artificial intelligence.

Until Tuesday, Gebru was a staff researcher and co-lead of an ethical AI team at Google.

Gebru tweeted what she said was the dismissal message from Google, which noted that "aspects of the email you sent last night to non-management employees in the brain group reflect behavior that is inconsistent with the expectations of a Google manager."

The research paper draft Gebru was told to retract looked at potential for an AI tool used by Google and other tech firms to imitate human writing of hate speech and biased language, according to an NPR report.

More than 1,000 Google employees were among the nearly 2,500 names on an online letter calling on the tech giant to explain Gebru's dismissal along with the reason for ordering her to withdraw her research.

The letter demanded Google make an "unequivocal" commitment to research integrity and academic freedom.

"Instead of being embraced by Google as an exceptionally talented and prolific contributor, Dr Gebru has faced defensiveness, racism, gaslighting, research censorship, and now a retaliatory firing," the letter contended.

Google did not respond to a request for comment.

Gebru, who is Ethiopian American, earned engineering degrees and a doctorate from Stanford University in Silicon Valley. She has worked at Apple and conducted research at Microsoft.

Gebru's research has included showing the likelihood of facial detection technology making mistakes when identifying people of color.

Her dismissal came just days after a US labor board complaint accused Google of using surveillance, interrogation and other tactics to spy on activist employees.

The National Labor Relations Board (NLRB) complaint stemmed from the dismissal a year ago of a quartet of employees dubbed the "Thanksgiving Four."

The workers sought a federal investigation into their dismissal, alleging they were sacked in retaliation for their labor organizing efforts, while Google maintained the employees had violated data security policies.