San Francisco (AFP) - Google on Monday fired four employees on the grounds they had violated data security policies, but the tech titan was accused of persecuting them for trying to unionize staff.

The dismissals of the quartet -- dubbed the "Thanksgiving Four" on social media -- deepened staff-management tensions at a company once seen as a paradigm of Silicon Valley freedoms but now embroiled in numerous controversies.

A memo to employees titled "Securing our data" sought to correct what Google contended was misinformation about the purported wrongdoing, saying it involved "systematic searches for other employees' materials and work."

Google, the money-making engine of parent company Alphabet, confirmed a copy of the note published by Bloomberg News but declined to comment further to AFP.

The memo said the information accessed by the individuals, "along with details of internal emails and inaccurate descriptions about Googlers’ work, was subsequently shared externally."

But the Tech Workers Coalition said the employees had been fired for "organizing at work" and urged others at Google to speak out.

"This is meant to scare workers, don't let it," the campaign group tweeted, appealing for other employers to step in and hire the four. At least one job offer came through in response.

One of the workers fired was connected to a petition condemning Google for working with the US customs and border patrol agency, which has been involved in President Donald Trump's crackdown on illegal immigration.

"Four of our colleagues took a stand and organized for a better workplace," read a Medium post by Google Walkout for Real Change, which organised a staff protest last year over the issue of sexual misconduct.

"This is explicitly condoned in Google's Code of Conduct, which ends: 'And remember ... don't be evil, and if you see something that you think isn't right — speak up.'

"When they did, Google retaliated against them."

Authors of the post contended that Google policies on data security were tightened simply to provide cover for getting rid of the workers.

"This is classic union busting dressed up in tech industry jargon, and we won't stand for it," they said.

- Global disputes -

The Google workplace has been disrupted by employee opposition to top-level decisions ranging from forging contracts with the US military to tailoring a version of the search engine for China.

A year ago, Google employees poured out of premises at its Mountain View campus and around the world to protest the company's handling of sexual misconduct allegations.

When they were still suspended prior to the firings, two of the quartet on Friday addressed a protest rally at the company's San Francisco offices, according to US media.

Jeffrey Hirsch, a University of North Carolina law professor and former National Labor Relations Board attorney, said Google could face legal problems unless it could show a consistent approach towards other staff accused of the same conduct as the four.

"If not, Google will likely have to reinstate the employees and pay them back pay," he told Bloomberg.

Google's virtual monopoly on internet searches has provoked accusations that it abuses its global dominance to attract more advertising money at the expense of others, such as the news media.

In France, an alliance of press groups is fighting back with a complaint under the European Union's new copyright law. AFP has brought a separate complaint against Google.

Like Facebook and Twitter, Google also stands accused of turning a blind eye to political disinformation on its platforms.

Last week, the search leader updated how it handles political ads, stepping up actions it says it is taking in the buildup to the US presidential election next year.