My previous organisation is not paying me my due salary/allowance

Hi, My previous organisation is not paying me my dues including my salary and other allowances and provided a negative feedback to my present employer. They did not verify my employment with them in order to restrict me from joining other organisation. Even after sending them multiple emails and calls they did not provide the desired feedback. And on asking for dues they sent an email stating that I have joined their client and have breached the NON DISCLOSURE agreement so they will not be paying me anything. But this is not true as I have joined some other organisation and they have deployed me with the same client as the client requested my present employer to deploy me with them because of my good work. This is not the first time my previous organisation is doing such thing with an employee as they have a past record of harassing its employees but those employees couldn't gather enough courage to stand against an organisation. My previous organisation is in Mumbai but my present location is Delhi. Please guide me on how to proceed further on this as it is impacting my career and health dearly.