Members of the GMB union staged demonstrations outside three restaurants from the six-strong Sofra chain in London last week over what they see as unfair tipping, and new contracts.
GMB organiser Raj Gill said staff had complained about a tips system that awarded most money to the managers and only a fraction to the restaurant staff.
The union said: "Workers, who have never received a contract before, are now being pressurised to sign new contracts that they are unhappy with. Some workers' hours have been reduced without proper consultation."
Gill added that the union would step up the pressure to secure an agreement with the restaurant chain.
A Sofra worker said staff at two restaurants were refusing to sign written contracts without union help to ensure all their working conditions were outlined clearly in the contract.
Erhan Barut, human resources manager at Sofra, denied that staff were under pressure to sign the contracts and said Sofra was simply meeting its legal obligations.
He added that hours had been cut because many of the workers are students or immigrants and had been working more hours than permitted by law.
Barut said the group had installed a new hourly salary scheme offering the national minimum wage plus 20p for each year's service, and launched a more even-handed distribution of tips based on hierarchy, years served and hours worked.
By Angela Frewin