Legal notice from my previous company for joining client company

Hi All, I was working for a company say XYZ and as part of the terms & conditions of the offer, I wasn't supposed to take up any whole time, part time employment or engage myself as a consultant directly or indirectly with my previous company's customers/clients for a period of 18 months from the date of termination of my employment with my previous company. After joining my new company, I wasn't aware of their clients, after joining the new company I was deputed for the same client after 10 months, which was the only option left for me. Now my previous company has forward me a legal notice demanding 7.5 lakh rupees including 5 lakh for the financial loss caused to them by my joining the same client and 2.5 lakh as a compensation for the hardship caused to my previous company due to my irresponsible behaviour & malafide intentions My question to all the respected seniors is, as part of the India labour law can an Individual be stopped working for the same client through a different company and how my joining a rival company caused financial loss of 5 lakh to my previous employer and how can they charge me 2.5 lakh as a compensation for my irresponsible behaviour. There is no break-up of the loss description. Does my previous employer has a valid case and can I justify my point, since the only option left for me was joining the new company & working for the same client, else i was left with no job options. I am not sure about this issue and I am getting different opinions from the different person. I served my notice period of 3 months before leaving my previous company. Please, help me with my concern. Regards