Google has been given two weeks to respond to a U.S. labor board complaint accusing the internet giant of using surveillance, interrogation and other tactics to spy on activist employees.

The National Labor Relations Board (NLRB) complaint filed late Wednesday stemmed from the dismissal a year ago of some 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 that the employees had violated data security policies.

A copy of the NLRB complaint contended that Google "surveilled" employees by peeking at slides being preparing in support of unionizing workers.

Google also "interrogated employees about protected activities" and threatened reprisals for venturing outside official company channels for handling complaints on such matters as workplace conduct, according to the complaint.

In addition, Google selectively applied rules to workers who participated in activist activities, it contended.

"It's vindicating," said Laurence Berland, one of the employees in the complaint. "It's become clear that Google's actions are not defensible, and not just legally."

Google has until December 16 to file an answer. A hearing is slated for April 12 in San Francisco.

"We strongly support the rights our employees have in the workplace, and open discussion and respectful debate have always been part of Google," said a Google spokesperson, adding that  "We're proud of our culture and committed to defending it against attempts by individuals to deliberately undermine it - including by violating security policies and internal systems."

Google added that it will continue to provide information to the NLRB about its decisions to terminate or discipline employees.

"Employees who speak out on ethical issues, harassment, discrimination and all these matters are no longer really welcome at Google in the way they used to be," said Berland,adding that "I think it is part of a shift in culture there."

(With input from AFP)