Hundreds of employees at Google and its parent company, Alphabet, have launched a union, a rare step for the tech industry that also represents the biggest and most organised challenge yet to the company's executive leadership.

Announced Monday morning, the Alphabet Workers' Union will be run by employees and open both to full-time workers and contractors at the tech giant.

'This is historic,' said Dylan Baker, a Google software engineer, in a release.

"The first union at a major tech company by and for all tech workers. We will elect representatives, we will make decisions democratically, we will pay dues, and we will hire skilled organisers to ensure all workers at Google know they can work with us if they actually want to see their company reflect their values."

The initiative launched with more than 200 members, according to a New York Times op-ed the group published Monday morning.

As many as 226 of the group's members have signed union cards with the Communications Workers of America, one of the country's largest labour unions.

Unions can be federally certified when a majority of a workplace votes to support one in an election held by the National Labor Relations Board.

Alternatively, according to the NLRB, a union can be voluntarily recognized by an employer if enough workers indicate they would like the union to negotiate on their behalf.

"We've always worked hard to create a supportive and rewarding workplace for our workforce," said Kara Silverstein, Google's director of people operations, in a statement to CNN Business.

"Of course our employees have protected labor rights that we support. But as we've always done, we'll continue engaging directly with all our employees."

The union's formation comes after years of rising employee tensions at Google over the company's business and operational decisions, including its work with the defense sector, plans for a censored search engine in China, and the company's handling of sexual misconduct claims, the last of which resulted in a massive employee protest.

The union's demands hit on many of those issues.

"For far too long, thousands of us at Google and other subsidiaries of Alphabet, Google's parent company have had our workplace concerns dismissed by executives," the workers wrote in the Times op-ed.

"Our bosses have collaborated with repressive governments around the world. They have developed artificial intelligence technology for use by the Department of Defense and profited from ads by a hate group.

They have failed to make the changes necessary to meaningfully address our retention issues with people of color.

"The organisers made special mention of Timnit Gebru, a top AI researcher who recently departed from Google after she criticised the company's diversity efforts sparking an outcry over Google's workplace practices.

Some workers who have raised their voices in the past have been terminated, in certain cases leading to complaints to the NLRB.

In December, the NLRB accused Google of breaking the law when it allegedly fired two employees for organizing.