Fifteen black former or current employees at Tesla filed a lawsuit against the electric car maker on Thursday, alleging they were subjected to racial abuse and harassment at its factories.

The workers said they were subjected to offensive racist comments and behaviour by colleagues, managers and human resources employees on a regular basis, according to the lawsuit filed in a California state court.

The harassment, which occurred mostly at Tesla's Fremont, California, factory, included using the n-word and such terms as "slavery" or "plantation" or making sexual comments such as "likes booty," the lawsuit said, adding that the automaker's "standard operating procedures include blatant, open and unmitigated race discrimination."

The filing described one plaintiff, Teri Mitchell, as being regularly harassed by co-workers and managers who used racial slurs and made statements including, "It is rare for Blacks to work here. I don't know how long you will be able to stay here."

Another plaintiff, Nathaniel Aziel Gonsalves, described harassment by a supervisor. The complaint described the supervisor as saying that "Gonsalves 'wasn't like most Black people,' that he 'didn't act ghetto,' and further called him a 'zebra' because he was 'neither black nor white.'"

Some of the plaintiffs were assigned to the most physically demanding posts in Tesla or passed over for promotion, according to the lawsuit.

It said that Montieco Justice, a production associate at Tesla's Fremont factory, was immediately demoted upon returning to Tesla after taking an authorised leave of absence as a result of contracting Covid-19.

Tesla did not respond to Reuters requests for comment on Thursday or Friday.

The automaker is facing at least 10 lawsuits alleging widespread race discrimination or sexual harassment, including one by a California state civil rights agency.

Tesla in February responded to the expected lawsuit by the California state agency, the Department of Fair Employment and Housing, saying it opposes discrimination and investigates all complaints.

"Tesla has always disciplined and terminated employees who engage in misconduct, including those who use racial slurs or harass others in different ways. We recently rolled out an additional training programme that reinforces Tesla’s requirement that all employees must treat each other with respect and reminds employees about the numerous ways they can report concerns, including anonymously," it said in a company blog.

On Monday, a federal judge in California ordered a new trial on the damages Tesla owes to a Black former factory worker who accused the company of race discrimination, after he turned down a $15 million award.

This month, a Tesla shareholder filed a lawsuit accusing Chief Executive Officer Elon Musk and the company's board of directors of neglecting worker complaints and fostering a toxic workplace culture.