Wal-Mart employs 1.1 million people in the USA

The workers, employed as janitors by subcontractors, say they were paid lower wages and offered fewer benefits.

The plaintiffs, who face deportation, were among 250 people arrested in an October immigration crackdown at 60 Wal-Mart stores in 21 states.

The nine's lawsuit states they are seeking more than $200,000 in back pay.

Overtime claim

Lawyer Gilberto Garcia said the lawsuit, filed in New Jersey Supreme Court, seeks to represent all the detained illegal immigrants who had worked for Wal-Mart.

They accuse Wal-Mart and its cleaning contractors of failing to pay for overtime, withhold taxes or make required workers' compensation contributions.

The plaintiffs say they worked at least 56 hours a week and were not paid time and a half for overtime, hours worked beyond 40 a week.

They say they were paid $350 to $500 a week.

The workers, mainly Eastern Europeans, were arrested by the US Immigration and Customs Enforcement (ICE) Bureau, in an investigation known as Operation Rollback.

'Nothing wrong'

Wal-Mart acknowledged it is the object of a federal investigation related to the employment of the undocumented workers.

But Mona Williams, Wal-Mart vice president of communications, said the company did not know about the alleged labor violations, or that the contractors used illegal immigrants.

She said Wal-Mart has long insisted that its contractors obey the law.

"We have seen absolutely no evidence showing that Wal-Mart did anything wrong", she said.

Wal-Mart, is the world's largest retailer and the largest private employer in the United States, with 1.1 million domestic employees and about 3,500 stores.